Review `if (fgGlobalMorph)` checks

As per https://github.com/dotnet/runtime/pull/122368#discussion_r2668225607, some paths in morph restrict optimizations to only running as part of `global morph` and does so to avoid having to deal with VN updates. We should investigate whether these checks can be removed and whether we can keep VN up to date even if morph is called later, or whether we can stop calling morph entirely after VN.