typeassert does not constrain types on subsequent path

[Keno]

function foo(x)
       a = x ? (1,2) : nothing
       a::Tuple
       a[1]
end

@code_warntype foo(true)
Variables:
  x::Bool
  a::Union{Nothing, Tuple{Int64,Int64}}
  #temp#@_4::Union{Nothing, Tuple{Int64,Int64}}
  #temp#@_5::Int64

Body:
  begin
      unless x::Bool goto 4
      #temp#@_4::Union{Nothing, Tuple{Int64,Int64}} = (1, 2)
      goto 6
      4:
      #temp#@_4::Union{Nothing, Tuple{Int64,Int64}} = Main.nothing
      6:
      a::Union{Nothing, Tuple{Int64,Int64}} = #temp#@_4::Union{Nothing, Tuple{Int64,Int64}}
      #= line 3 =#
      (Core.typeassert)(a::Union{Nothing, Tuple{Int64,Int64}}, Main.Tuple)::Tuple{Int64,Int64}
      #= line 4 =#
      SSAValue(1) = (a::Union{Nothing, Tuple{Int64,Int64}} isa Tuple{Int64,Int64})::Bool
      unless SSAValue(1) goto 15
      #temp#@_5::Int64 = $(Expr(:invoke, MethodInstance for getindex(::Tuple{Int64,Int64}, ::Int64), :(Main.getindex), :(a), 1))::Int64
      goto 20
      15:
      goto 17
      17:
      #temp#@_5::Int64 = (Main.getindex)(a::Union{Nothing, Tuple{Int64,Int64}}, 1)::Int64
      goto 20
      20:
      SSAValue(0) = #temp#@_5::Int64
      return SSAValue(0)
  end::Int64

There should be a TypedSlot on the getindex since we know that after the typeassert a can't be nothing.

[vtjnash]

Inference was fixed by #38905, but the optimizer still generates poor code here (it deletes the PiNode required for codegen to be able to use the inference result)