[llvm] AMDGPU: Add test showing bit operations that should be reducible (PR #141837)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jun 10, 2:36 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/141837).


https://github.com/llvm/llvm-project/pull/141837