[llvm] [AMDGPU][SIInsertWaitCnts] De-duplicate code (NFC) (PR #161161)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

### Merge activity

* **Oct 1, 8:51 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/161161).


https://github.com/llvm/llvm-project/pull/161161