[llvm] [AMDGPU] Canonicalize G_ZEXT of the shift amount in RegBankCombiner (PR #131792)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

### Merge activity

* **May 14, 4:46 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/131792).


https://github.com/llvm/llvm-project/pull/131792