[CodeGen] Handle dead defs in LivePhysRegs::stepForward

[foad]

Summary

LivePhysRegs::stepForward already handled killed uses by removing them
from the set of live registers. This patch makes it handle dead defs in
the same way.

This does not affect any current tests but is required for a forthcoming
change to use LivePhysRegs in ExpandPostRAPseudos.

llvm/lib/CodeGen/LivePhysRegs.cpp

void LivePhysRegs::stepForward(const MachineInstr &MI,
   for (ConstMIBundleOperands O(MI); O.isValid(); ++O) {
     if (O->isReg()) {
       if (O->isDebug())
         continue;
       Register Reg = O->getReg();
       if (!Reg.isPhysical())
         continue;
       if (O->isDef()) {
+        if (O->isDead())
+          removeReg(Reg);
         // Note, dead defs are still recorded.  The caller should decide how to
         // handle them.
         Clobbers.push_back(std::make_pair(Reg, &*O));
       } else {
         assert(O->isUse());
         if (O->isKill())
           removeReg(Reg);
       }