[llvm] [IRCE] Check loop clone legality before attempting to do so (PR #151062)

[Keno Fischer via llvm-commits]

Keno wrote:

> Though I would note the description makes it sound NFC for the existing loop unstitching, but it looks like you also improved the used outside BB check to be a more permissive used outside loop check, which should perhaps be noted in the commit message

Yes, that's correct, since I also unified the logic with isBlockInLCSSAForm and that's what I wanted for IRCE to fix this bug (while not pessimizing unnecessarily for uses inside the loop).

https://github.com/llvm/llvm-project/pull/151062