[llvm] AMDGPU: Add tests with forced selection of mfmas to VGPR form (PR #150626)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jul 25, 4:01 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/150626).


https://github.com/llvm/llvm-project/pull/150626