[llvm] [InstCombine] Fold xor-lshr sign-mask idiom to ashr (PR #213954)

[via llvm-commits]

Andarwinux wrote:

> @Andarwinux, thank you for assigning #204851 to me. Would you be willing to review this PR?

I only have triage permission, so I can't participate in review. But I can add some reviewers for you if you need.

https://github.com/llvm/llvm-project/pull/213954