[llvm] AMDGPU: Add baseline tests for simplify elts of readfirstlane (PR #128645)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Feb 28, 12:52 AM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/128645).


https://github.com/llvm/llvm-project/pull/128645