[llvm] AMDGPU/NewPM: Start implementing addCodeGenPrepare (PR #102816)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Aug 19, 4:09 PM EDT**: @arsenm started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/102816).


https://github.com/llvm/llvm-project/pull/102816