[llvm] [SCEV] Speed up forgetLoop by avoiding def-use walk for loop-header PHIs (PR #201572)

[Nikita Popov via llvm-commits]

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

LGTM. I'm not very confident in this change, but I don't have any specific counter-examples in mind. We can give this a try and see what falls out...

https://github.com/llvm/llvm-project/pull/201572