[llvm] AMDGPU: Add V_CVT_F32_BF16 for gfx950 (PR #116311)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Nov 18, 4:29 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/116311).


https://github.com/llvm/llvm-project/pull/116311