[llvm] AMDGPU: Replace undef with poison in tests using insertvalue (PR #130895)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Mar 12, 5:10 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/130895).


https://github.com/llvm/llvm-project/pull/130895