[ModuloSchedule] Support instructions with > 1 destination when walking canonical use.

[hgreving]

Summary

Fixes a minor bug that led to finding the wrong register if the definition had more
than one register destination.

llvm/lib/CodeGen/ModuloSchedule.cpp

void PeelingModuloScheduleExpander::moveStageBetweenBlocks(
   }
 }
 
 Register
 PeelingModuloScheduleExpander::getPhiCanonicalReg(MachineInstr *CanonicalPhi,
                                                   MachineInstr *Phi) {
   unsigned distance = PhiNodeLoopIteration[Phi];
   MachineInstr *CanonicalUse = CanonicalPhi;
+  Register CanonicalUseReg = CanonicalUse->getOperand(0).getReg();
   for (unsigned I = 0; I < distance; ++I) {
     assert(CanonicalUse->isPHI());
     assert(CanonicalUse->getNumOperands() == 5);
     unsigned LoopRegIdx = 3, InitRegIdx = 1;
     if (CanonicalUse->getOperand(2).getMBB() == CanonicalUse->getParent())
       std::swap(LoopRegIdx, InitRegIdx);
-    CanonicalUse =
-        MRI.getVRegDef(CanonicalUse->getOperand(LoopRegIdx).getReg());
+    CanonicalUseReg = CanonicalUse->getOperand(LoopRegIdx).getReg();
+    CanonicalUse = MRI.getVRegDef(CanonicalUseReg);
   }
-  return CanonicalUse->getOperand(0).getReg();
+  return CanonicalUseReg;
 }
 
 void PeelingModuloScheduleExpander::peelPrologAndEpilogs() {
   BitVector LS(Schedule.getNumStages(), true);
   BitVector AS(Schedule.getNumStages(), true);
   LiveStages[BB] = LS;
   AvailableStages[BB] = AS;