[llvm] AMDGPU: Add baseline test for coalescing vgpr to agpr subreg inserts (PR #130877)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Mar 11, 11:06 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/130877).


https://github.com/llvm/llvm-project/pull/130877