useless rettype_const stored

It'd be nice if we didn't feel the need to store an extra array on every CodeInstance that we inferred allocates something reminding us that we don't know what exactly it will have allocated. This is a (memory) regression in v1.12
```
julia> which(Base.RefValue{Any}, (Any,)).specializations[1].cache.rettype_const
1-element Vector{Any}:
 Int32
```