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

[Ningning Shi 史宁宁 via llvm-commits]

================
@@ -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
----------------
shining1984 wrote:

> The convention for new code is "Don’t duplicate function or class name at the beginning of the comment." https://llvm.org/docs/CodingStandards.html#commenting

This pr just fixs three existed comments.
There are many comments in the two files need to update, if follow the   "Don’t duplicate function or class name at the beginning of the comment." Maybe need a new pr to update them.

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