[llvm] [SCEV] Improve code in SCEVLoopGuardRewriter (NFC) (PR #139257)

[Florian Hahn via llvm-commits]

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

LGTM, thanks. 

Might be also worth folding the `lookup` into the `if` if possible

https://github.com/llvm/llvm-project/pull/139257