[llvm] [AArch64] Signed comparison using CMN is safe when the subtraction is nsw (PR #141993)

[David Green via llvm-commits]

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

Thanks - I think I convinced myself that this was OK. LGTM

Are you happy for it to be submitted?

https://github.com/llvm/llvm-project/pull/141993