MachineInstr: addRegisterDefReadUndef() => setRegisterDefReadUndef()

This way we can not only add but also remove read undef flags.

llvm-svn: 252678
diff --git a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
index afd8db8..9451d92 100644
--- a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
+++ b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
@@ -490,7 +490,7 @@
     if (MRI->shouldTrackSubRegLiveness(VReg)) {
       if ((I == LI.begin() || std::prev(I)->end < Def) && !VNI->isPHIDef()) {
         MachineInstr *MI = getInstructionFromIndex(Def);
-        MI->addRegisterDefReadUndef(VReg);
+        MI->setRegisterDefReadUndef(VReg);
         DeadBeforeDef = true;
       }
     }