[llvm] AMDGPU: Directly check if shrink-instructions run is post-RA (PR #142009)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **May 30, 1:50 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/142009).


https://github.com/llvm/llvm-project/pull/142009