[llvm] [SCEV] Use loop guards when checking that RHS >= Start (PR #75039)

[Florian Hahn via llvm-commits]

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

LGTM, thanks assuming the compile-time impact isn't too bad! 

Using loop guards throughout the various `howMany*` would probably be beneficial in general, but that would probably need some sort of caching of the collected conds to keep compile-time impact low

https://github.com/llvm/llvm-project/pull/75039