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

[Jay Foad via llvm-commits]

jayfoad wrote:

* **#105549** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/105549?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a>
* **#105548** <a href="https://app.graphite.dev/github/pr/llvm/llvm-project/105548?utm_source=stack-comment-icon" target="_blank"><img src="https://static.graphite.dev/graphite-32x32-black.png" alt="Graphite" width="10px" height="10px"/></a> 👈
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https://github.com/llvm/llvm-project/pull/105548