[llvm] AMDGPU: Replace insertelement poison with insertelement undef (PR #130896)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Mar 12, 9:29 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/130896).


https://github.com/llvm/llvm-project/pull/130896