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

[via llvm-commits]

AZero13 wrote:

@topperc @dtcxzyw Actually, now that I think about it, if I check for nsw, is the Knownbits needed at all?

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