[CodeGen][NFC] Guard copy propogation in machine CSE against undefs

Author: vikramRH

llvm/lib/CodeGen/MachineCSE.cpp

@@ -184,7 +184,7 @@ bool MachineCSE::PerformTrivialCopyPropagation(MachineInstr *MI,
       continue;
     bool OnlyOneUse = MRI->hasOneNonDBGUse(Reg);
     MachineInstr *DefMI = MRI->getVRegDef(Reg);
-    if (!DefMI->isCopy())
+    if (!DefMI || !DefMI->isCopy())
       continue;
     Register SrcReg = DefMI->getOperand(1).getReg();
     if (!SrcReg.isVirtual())