[llvm] [MachineScheduler][AMDGPU] Allow scheduling of single-MI regions (PR #128739)

[Matt Arsenault via llvm-commits]

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@@ -780,7 +780,7 @@ void MachineSchedulerBase::scheduleRegions(ScheduleDAGInstrs &Scheduler,
       Scheduler.enterRegion(&*MBB, I, RegionEnd, NumRegionInstrs);
 
       // Skip empty scheduling regions (0 or 1 schedulable instructions).
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arsenm wrote:

Adjust the comment to explain why this shouldn't skip 1 sized regions. Also adjust the patch description 

https://github.com/llvm/llvm-project/pull/128739