[llvm] AMDGPU: Use poison in some lowering contexts (PR #137325)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Apr 25, 10:01 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/137325).


https://github.com/llvm/llvm-project/pull/137325