[llvm] [AArch64] Use isKnownNonZero to optimize eligible compares to cmn (PR #96349)

[via llvm-commits]

AtariDreams wrote:

> I think this looks OK now, and the test all look OK. Do you mind if we wait until after the release branch to commit, in case there is something still wrong?
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> It would be good to have test cases for lots of various cases too, even if they don't trigger - different signed conditions, cmn on the LHS/RHS, ccmn on the LHS/RHS etc. Thanks

If something goes wrong, we can revert it. However, I've tested and tested this and found no issue. 



https://github.com/llvm/llvm-project/pull/96349