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

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

Compile-time impact looks on track now: https://llvm-compile-time-tracker.com/compare.php?from=9d7df23f4d6537752854d54b0c4c583512b930d0&to=66fa76366674b0fb7e7c270b452796c9c2376fab&stat=instructions:u.

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