[llvm] [AMDGPU][Scheduler] Make `finalizeGCNRegion` an overridable hook (NFC) (PR #177199)

[Lucas Ramirez via llvm-commits]

lucas-rami wrote:

"Finalize" may not be a super clear name indeed; even in the current implementation this is where re-scheduling is reverted if it turns out to be worse than before (stages can customize what "worse" means with the `shouldRevertScheduling` hook) so stages already have some power to control what happens after a region is re-scheduled. `finalizeGCNSchedStage` is another stage hook that is invoked after all regions have been re-scheduled.

https://github.com/llvm/llvm-project/pull/177199