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

[Nikita Popov via llvm-commits]

nikic wrote:

Why is your ctlz fold generating xor in the first place, instead of sub? Your xor implementation is a miscompile for non-pow2 bws, sub is always correct.

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