[llvm] [CodeGen][MachineScheduler][NFC]Update some comments of scheduler (PR #74705)

[Wang Pengcheng via llvm-commits]

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@@ -747,9 +747,9 @@ void ScheduleDAGMI::finishBlock() {
   ScheduleDAGInstrs::finishBlock();
 }
 
-/// enterRegion - Called back from MachineScheduler::runOnMachineFunction after
-/// crossing a scheduling boundary. [begin, end) includes all instructions in
-/// the region, including the boundary itself and single-instruction regions
+/// enterRegion - Called back from PostMachineScheduler::runOnMachineFunction
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wangpc-pp wrote:

Is this true? I think both `PostMachineScheduler` and `MachineScheduler` call this.
Should it be `MachineSchedulerBase::scheduleRegions`?

https://github.com/llvm/llvm-project/pull/74705