[llvm] AMDGPU: Replace ptr addrspace(4) undef uses with poison in tests (PR #131095)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Mar 13, 10:46 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/131095).


https://github.com/llvm/llvm-project/pull/131095