[llvm] AMDGPU: Add baseline test for lane index simplification (PR #117962)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Dec 2, 2:49 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/117962).


https://github.com/llvm/llvm-project/pull/117962