[llvm] [InstCombine] Canonicalize `(sub nuw Mask, X)` -> `(xor Mask, X)` (PR #122542)

[via llvm-commits]

goldsteinn wrote:

> The inability to fold the `xor` version w/ `add` leads to many regressions. OTOH there are definetly some positive cases.
> 
> I think we want to either just make this a simplification or we need some IR flag indicating the equivilence.

Or just update folds using `xor` to also accept `sub nuw`. Let me know which of these sounds most reasonable.

https://github.com/llvm/llvm-project/pull/122542