[AMDGPU] Fix PreRARematStage::sinkTriviallyRematInsts region boundary update after sinking.

[vpykhtin]

Summary

First boundary of a region wasn't updated when a sinked instruction was added first into the region.

[vangthao]

LGTM , thanks!

[vpykhtin]

rebase

[arsenm]

Needs test

llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp

for (auto &It : RematerializableInsts[I]) {
           MachineBasicBlock::iterator(It.second);
       Register Reg = Def->getOperand(0).getReg();
       // Rematerialize MI to its use block. Since we are only rematerializing
       // instructions that do not have any virtual reg uses, we do not need to
       // call LiveRangeEdit::allUsesAvailableAt() and
       // LiveRangeEdit::canRematerializeAt().
       TII->reMaterialize(*InsertPos->getParent(), InsertPos, Reg,
                          Def->getOperand(0).getSubReg(), *Def, *DAG.TRI);
-      MachineInstr *NewMI = &*(--InsertPos);
+      MachineInstr *NewMI = &*std::prev(InsertPos);
       LIS->InsertMachineInstrInMaps(*NewMI);
       LIS->removeInterval(Reg);
       LIS->createAndComputeVirtRegInterval(Reg);
       InsertedMIToOldDef[NewMI] = Def;
 
       // Update region boundaries in scheduling region we sinked from since we
       // may sink an instruction that was at the beginning or end of its region
       DAG.updateRegionBoundaries(NewRegions, Def, /*NewMI =*/nullptr,