[llvm] AMDGPU: Use freeze poison instead of undef in alloca promotion (PR #131285)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Mar 18, 6:23 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/131285).


https://github.com/llvm/llvm-project/pull/131285