[llvm] AMDGPU: Migrate more tests away from undef (PR #131314)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Mar 17, 4:47 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/131314).


https://github.com/llvm/llvm-project/pull/131314