[CodeGen} Avoid handling DBG_VALUE in LiveRegUnits::stepBackward

[JesperAntonsson]

Summary

This patch prevents DBG_VALUE instructions from influencing LiveRegUnits::stepBackward(). It mirrors rL327862 / D43850 that made the same changes for LivePhysRegs.

What motivated this patch was a problem in an out-of-tree target where the PrologEpilogInserter does register scavenging and decides it has to spill the scavenged register that LiveRegUnits finds live as it has a DBG_VALUE use. Since it's out-of-tree and a bit involved, I don't have a testcase to present here.

[JesperAntonsson]

Thanks Krzysztof. I don't have commit rights yet, so I'd appreciate help to get this merged to trunk.

lib/CodeGen/LiveRegUnits.cpp

for (unsigned U = 0, E = TRI->getNumRegUnits(); U != E; ++U) {
     }
   }
 }
 
 void LiveRegUnits::stepBackward(const MachineInstr &MI) {
   // Remove defined registers and regmask kills from the set.
   for (ConstMIBundleOperands O(MI); O.isValid(); ++O) {
     if (O->isReg()) {
-      if (!O->isDef())
+      if (!O->isDef() || O->isDebug())
         continue;
       unsigned Reg = O->getReg();
       if (!TargetRegisterInfo::isPhysicalRegister(Reg))
         continue;
       removeReg(Reg);
     } else if (O->isRegMask())
       removeRegsNotPreserved(O->getRegMask());
   }
 
   // Add uses to the set.
   for (ConstMIBundleOperands O(MI); O.isValid(); ++O) {
-    if (!O->isReg() || !O->readsReg())
+    if (!O->isReg() || !O->readsReg() || O->isDebug())
       continue;
     unsigned Reg = O->getReg();
     if (!TargetRegisterInfo::isPhysicalRegister(Reg))
       continue;
     addReg(Reg);
   }
 }