[llvm] Port isNonZeroShift to SelectionDAG (PR #146125)

[Nikita Popov via llvm-commits]

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

Please ensure that any non-draft PRs you submit have an adequate PR description and test coverage **at the time of submission**.

(I will not review this beyond this comment. Porting this special case to SDAG is likely useless and not worth our time.)

https://github.com/llvm/llvm-project/pull/146125