[llvm] [AMDGPU] Add GFX12 test coverage for vmcnt flushing in loop headers (PR #105548)

[Jay Foad via llvm-commits]

jayfoad wrote:

### Merge activity

* **Aug 22, 6:34 AM EDT**: @jayfoad started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/105548).


https://github.com/llvm/llvm-project/pull/105548