[llvm] [NFC][AMDGPU] Add an IR test for `v_cvt_f16_bf8` (PR #149627)

[Shilei Tian via llvm-commits]

shiltian wrote:

### Merge activity

* **Jul 21, 2:51 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/149627).


https://github.com/llvm/llvm-project/pull/149627