[llvm] Revert "[SCEV] Speed up forgetLoop by avoiding def-use walk for loop-header PHIs" (PR #212485)

[Antonio Frighetto via llvm-commits]

https://github.com/antoniofrighetto approved this pull request.

A bit unfortunate to sacrifice the compile-time improvement ([c-t-t](https://llvm-compile-time-tracker.com/compare.php?from=708dafb270b5f6971c6666cc0afe96d227fb198d&to=c99f5b6edbca9497f328712aff907f0345b07a0b&stat=instructions:u)), hopefully we can revisit once the direction is a bit clearer.

https://github.com/llvm/llvm-project/pull/212485