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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

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

We can start with adding a matcher for both Xor and `sub nuw MaskC, X`.

> we need some IR flag indicating the equivilence.

The downside is that we have to fight with more flag preservation bugs :(



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