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

[Jameson Nash via llvm-commits]

vtjnash wrote:

LGTM. 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

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