JIT: extend indirect virtual stub optimization to arrays

[AndyAyersMS]

Add support for array interface methods invoked via indirect virtual stubs. Trim out a layer of temps in the importer. Remember the base method handle and use that for detecting the GetEnumerator pattern and for array interface method devirtualization. Mark the SZGenericArrayEnumerator ctor as aggressively inlined; when inlined we can often prove the enumerator (conditionally) doesn't escape.

Part of #108913

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[AndyAyersMS]

@EgorBot -intel -arm64

using System.Collections.Generic;
using System.Runtime.CompilerServices;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

[GenericTypeArguments(typeof(string))]
[MemoryDiagnoser]
public class A<T>
{
    private IEnumerable<T> _ienumerable;

    [GlobalSetup(Target = nameof(IEnumerable))]
    public void SetupIEnumerable() => _ienumerable = new string[512] as IEnumerable<T>;

    [Benchmark]
    public T IEnumerable() => Get(_ienumerable);

    [MethodImpl(MethodImplOptions.NoInlining)]
    private T Get(IEnumerable<T> collection)
    {
        T result = default;
        foreach (var item in collection)
            result = item;
        return result;
    }

    private IEnumerable<string> _ienumerable_str;

    [GlobalSetup(Target = nameof(IEnumerable_str))]
    public void SetupIEnumerable_str() => _ienumerable_str = new string[512];

    [Benchmark]
    public string IEnumerable_str() => Get_str(_ienumerable_str);

    [MethodImpl(MethodImplOptions.NoInlining)]
    private string Get_str(IEnumerable<string> collection)
    {
        string result = default;
        foreach (var item in collection)
            result = item;
        return result;
    }
}

[AndyAyersMS]

Two failures that I spotted in scanning:

Unexpected NREs

    System.Collections.Tests.OrderedDictionary_Keys_IList_Generic_Tests.IEnumerable_Generic_GetEnumerator_NoExceptionsWhileGetting(count: 0) [FAIL]
      System.NullReferenceException : Object reference not set to an instance of an object.
      Stack Trace:
        /_/src/libraries/Common/src/System/Collections/Generic/EnumerableHelpers.cs(19,0): at System.Collections.Generic.EnumerableHelpers.GetEmptyEnumerator[T]()
        /_/src/libraries/System.Collections/src/System/Collections/Generic/OrderedDictionary.cs(1596,0): at System.Collections.Generic.OrderedDictionary`2.KeyCollection.System.Collections.Generic.IEnumerable<TKey>.GetEnumerator()
        /_/src/libraries/Common/tests/System/Collections/IEnumerable.Generic.Tests.cs(347,0): at System.Collections.Tests.IEnumerable_Generic_Tests`1.IEnumerable_Generic_GetEnumerator_NoExceptionsWhileGetting(Int32 count)
           at System.RuntimeMethodHandle.InvokeMethod(ObjectHandleOnStack target, Void** arguments, ObjectHandleOnStack sig, BOOL isConstructor, ObjectHandleOnStack result)
        /_/src/libraries/System.Private.CoreLib/src/System/Reflection/MethodBaseInvoker.cs(174,0): at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr)
    System.Collections.Tests.OrderedDictionary_Keys_IList_Generic_Tests.IEnumerable_Generic_Enumerator_Current_AfterEndOfEnumerable_UndefinedBehavior(count: 0) [FAIL]
      System.NullReferenceException : Object reference not set to an instance of an object.

A runtime assert

Assert failure(PID 6856 [0x00001ac8], Thread: 5192 [0x1448]): Consistency check failed: 
FAILED: (!implSlot.IsNull() || pMT->IsInterface()) && "Valid method implementation was not found."

[AndyAyersMS]

For the assert (here looking at the span Indexer test) we have this tree:

**** Late devirt opportunity
               [000157] --C-G------                         *  CALLV stub ref    System.Collections.Generic.IEnumerable`1[System.__Canon]:GetEnumerator():System.Collections.Generic.IEnumerator`1[System.__Canon]:this
               [000820] --CXG------ this                    \--*  CALL help ref    CORINFO_HELP_CHKCASTANY
               [000819] H------N--- arg0                       +--*  CNS_INT(h) long   0x7ffe0b544f48 class System.Collections.Generic.IEnumerable`1[Span.InlineDataAttribute]
               [000830] --C-G------ arg1                       \--*  CALL      ref    System.Attribute:GetCustomAttributes(System.Reflection.MemberInfo,System.Type,bool):System.Attribute[]
               [000145] ----------- arg0                          +--*  LCL_VAR   ref    V18 tmp5         
               [000827] --C-G------ arg1                          +--*  CALL help ref    CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPE
               [000828] H---------- arg0                          |  \--*  CNS_INT(h) long   0x7ffe0b5441d0 class Span.InlineDataAttribute
               [000829] ----------- arg2                          \--*  CNS_INT   int    1

We try late devirt, and need the type of this, so we invoke gtGetClassHandle([820]) .. this rejects the cast-to type as we try and avoid returning interface types, and uses the cast base type. So we devirt against Attribute[] instead of InlineDataAttribute[].

The devirt is to an instantiating stub, so we end up producing

               [000157] --C-G------                         *  CALL nullcheck ref    System.SZArrayHelper:GetEnumerator[System.__Canon]():System.Collections.Generic.IEnumerator`1[System.__Canon]:this
               [000820] --CXG------ this                    +--*  CALL help ref    CORINFO_HELP_CHKCASTANY
               [000819] H------N--- arg0                    |  +--*  CNS_INT(h) long   0x7ffe0b544f48 class System.Collections.Generic.IEnumerable`1[Span.InlineDataAttribute]
               [000830] --C-G------ arg1                    |  \--*  CALL      ref    System.Attribute:GetCustomAttributes(System.Reflection.MemberInfo,System.Type,bool):System.Attribute[]
               [000145] ----------- arg0                    |     +--*  LCL_VAR   ref    V18 tmp5         
               [000827] --C-G------ arg1                    |     +--*  CALL help ref    CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPE
               [000828] H---------- arg0                    |     |  \--*  CNS_INT(h) long   0x7ffe0b5441d0 class Span.InlineDataAttribute
               [000829] ----------- arg2                    |     \--*  CNS_INT   int    1
               [000832] H---------- gctx                    \--*  CNS_INT(h) long   0x7ffe0b5caa68 method System.SZArrayHelper:GetEnumerator[System.Attribute]():System.Collections.Generic.IEnumerator`1[System.Attribute]:this

and this context is the wrong type. So we do the wrong instantiation...(?) and blow up later trying to invoke MoveNext on this enumerator.

Maybe in array interface cases if the object (array) type is inexact we need to just bail out on devirtualization... let me see how messy that ends up being.