[CodeGen] Clear InitUndef pass new register cache between pass runs

Multiple invocations of the pass could interfere with eachother,
preventing some undefs being initialised.

Author: joe-img

llvm/lib/CodeGen/InitUndef.cpp

@@ -272,6 +272,7 @@ bool InitUndef::runOnMachineFunction(MachineFunction &MF) {
   for (auto *DeadMI : DeadInsts)
     DeadMI->eraseFromParent();
   DeadInsts.clear();
+  NewRegs.clear();
 
   return Changed;
 }