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

[Nikita Popov via llvm-commits]

nikic wrote:

It's not expected to be strictly NFC, because less things are invalidated, which may produce different results e.g. due to query order dependence.

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