[llvm] [X86] Select standalone ~(-1 << n) masks as BZHI (PR #226158)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

Make sure you add new tests in the first commit with baseline codegen, so that later commits in the PR chain show the codegen diff

https://github.com/llvm/llvm-project/pull/226158