[llvm] AMDGPU/GlobalISel: Insert m0 initialization before sextload/zextload (PR #111720)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Oct 10, 5:59 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/111720).


https://github.com/llvm/llvm-project/pull/111720