[SCCP] Go to overdef for undef constants. (WIP)

llvm/lib/Transforms/Scalar/SCCP.cpp

std::pair<DenseMap<Value*, LatticeVal>::iterator, bool> I =
       ValueState.insert(std::make_pair(V, LatticeVal()));
     LatticeVal &LV = I.first->second;
 
     if (!I.second)
       return LV;  // Common case, already in the map.
 
     if (auto *C = dyn_cast<Constant>(V)) {
       // Undef values remain unknown.
-      if (!isa<UndefValue>(V))
+      if (isa<UndefValue>(V))
+        LV.markOverdefined();
+      else
         LV.markConstant(C);          // Constants are constant
     }
 
     // All others are underdefined by default.
     return LV;
   }
 
   LatticeVal toLatticeVal(const ValueLatticeElement &V, Type *T) {
void SCCPSolver::visitPHINode(PHINode &PN) {
   for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
     LatticeVal IV = getValueState(PN.getIncomingValue(i));
     if (IV.isUnknown()) continue;  // Doesn't influence PHI node.
 
     if (!isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent()))
       continue;
 
     LatticeVal &Res = getValueState(&PN);
+    if (isa<UndefValue>(PN.getIncomingValue(i))) {
+      if (isConstant(Res))
+        continue;
+    }
     Changed |= Res.mergeIn(IV, DL);
     if (Res.isOverdefined())
       break;
   }
   if (Changed)
     pushToWorkListMsg(ValueState[&PN], &PN);
 }