[llvm] AMDGPU: Delete FillMFMAShadowMutation (PR #123861)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jan 22, 10:40 AM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/123861).


https://github.com/llvm/llvm-project/pull/123861