[llvm] [LV] Fix unsafe canVectorizeWithIfConvert() checks (PR #203273)

[Graham Hunter via llvm-commits]

https://github.com/huntergr-arm approved this pull request.

LGTM. We would normally reject in `canVectorizeLoopCFG`, but the extra analysis lets us explore past the initial problems and assumptions are made about the state of the loop. Confirmed a crash with godbolt.

https://github.com/llvm/llvm-project/pull/203273