[llvm] AMDGPU: Migrate some tests away from undef (PR #131277)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Mar 14, 1:27 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/131277).


https://github.com/llvm/llvm-project/pull/131277