[llvm] AMDGPU: Fix atomic expand tests accidentally underaligning (PR #147299)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jul 8, 2:57 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/147299).


https://github.com/llvm/llvm-project/pull/147299