[llvm] [AMDGPU] Add support for `v_cvt_pk_bf16_f32` on gfx1250 (PR #150053)

[Shilei Tian via llvm-commits]

shiltian wrote:

### Merge activity

* **Jul 22, 11:55 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/150053).


https://github.com/llvm/llvm-project/pull/150053