[llvm] Reapply "[GVN] MemorySSA for GVN: add optional AllowMemorySSA" (PR #122482)

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

I think we still need MemSSA as being used while doing some PRE as part of MergeBlockIntoPredecessor, which is coherent with the current code. Updated and dropped the assert for now. The change will be squashed into a single commit.

https://github.com/llvm/llvm-project/pull/122482