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

[Matt Arsenault via llvm-commits]

arsenm wrote:

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

This isn't really the point of adding tests. 

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