[llvm] AMDGPU: Add codegen support for gfx950 v_ashr_pk_i8/u8_i32 (PR #118304)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Dec 2, 7:21 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/118304).


https://github.com/llvm/llvm-project/pull/118304