[llvm] [AMDGPU] Cluster export instruction in PostRA Scheduler (PR #141399)

[Carl Ritson via llvm-commits]

perlfu wrote:

> Maybe this can only apply post-RA?

I think we do need to do this pre-RA as well so RA understands the liveness of the register used for the exports.

https://github.com/llvm/llvm-project/pull/141399