[ScheduleDAG][NFC] Use MCInstrDesc API to check PhysReg definition.

[dantrushin]

Summary

Use API provided by MCInstrDesc to check if instruction implicitly
defines physical register instead of direct unchecked access to
MCInstrDesc::ImplicitDefs array (which might be null).
This makes CheckForPhysRegDependency() more robust.

[dantrushin]

This was landed as part of larger patch (D81648) for which this was a blocker. Abandoning this review.

llvm/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp

if (Register::isVirtualRegister(Reg))
     return;
 
   unsigned ResNo = User->getOperand(2).getResNo();
   if (Def->getOpcode() == ISD::CopyFromReg &&
       cast<RegisterSDNode>(Def->getOperand(1))->getReg() == Reg) {
     PhysReg = Reg;
   } else if (Def->isMachineOpcode()) {
     const MCInstrDesc &II = TII->get(Def->getMachineOpcode());
-    if (ResNo >= II.getNumDefs() &&
-        II.ImplicitDefs[ResNo - II.getNumDefs()] == Reg)
+    if (ResNo >= II.getNumDefs() && II.hasImplicitDefOfPhysReg(Reg))
       PhysReg = Reg;
   }
 
   if (PhysReg != 0) {
     const TargetRegisterClass *RC =
         TRI->getMinimalPhysRegClass(Reg, Def->getSimpleValueType(ResNo));
     Cost = RC->getCopyCost();
   }