[llvm] AMDGPU: Add amdgpu-agpr-alloc attribute to control AGPR allocation (PR #128034)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Mar 5, 9:11 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/128034).


https://github.com/llvm/llvm-project/pull/128034