runtime: Fix type mismatches in VN

Value numbering can see through some forms of aliasing, but does not always account for type mismatches properly. One example of an incorrect behavior can be seen here: https://github.com/dotnet/runtime/pull/58309#issuecomment-907614688, there may be others.

Problematic places need to be investigated and fixed. A short-term solution could be to give up and give a “new, unique” VN upon seeing such casts, long-term we should consider using GT_BITCAST as a VNFunc for them, with support for folding (and delete assertion propagation code that expects these type mismatches), since, for better or worse, we have a strongly typed model in VN.

category:correctness theme:value-numbering skill-level:expert cost:small impact:small