[llvm] [CodeGen] Finish untangling LRE::scanRemattable [nfc] (PR #161963)

[Philip Reames via llvm-commits]

preames wrote:

Doesn't look like much compile time impact if any: https://llvm-compile-time-tracker.com/compare.php?from=25126117b5781e96453e5c5b1a9a6a6f8aa3989c&to=fe927580c21af14d706275f2859fd3ba6f9718cc&stat=instructions:u

https://github.com/llvm/llvm-project/pull/161963