[llvm] AMDGPU: Fold copy of scalar add of frame index (PR #115058)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Nov 6, 12:09 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/115058).


https://github.com/llvm/llvm-project/pull/115058