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

[via llvm-commits]

AZero13 wrote:

I fixed a comment typo, build failed for unrelated reason, so I just rebased just to ensure everything is good. Everything is good. We can merge.

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