[llvm] AMDGPU: Add baseline tests for cmpxchg custom expansion (PR #109408)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Oct 31, 2:43 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/109408).


https://github.com/llvm/llvm-project/pull/109408