[llvm] [DAGCombiner] Fold `sub nuw C, x` -> `xor x, C` when C is a mask (PR #192692)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

(style) split the commit in the PR so the first commit adds the baseline tests and the second commit adds the fold and shows the test codegen changes

Either that or split the new test into a new pre-commit pr

https://github.com/llvm/llvm-project/pull/192692