When folding a def to its use, need to make sure that use has that def as its single def.

Otherwise, the use cannot be folded to.

Author: jgu222

visa/Optimizer.cpp

@@ -4467,12 +4467,16 @@ bool Optimizer::foldPseudoNot(G4_BB* bb, INST_LIST_ITER& iter)
         auto&& use = *uses;
         G4_INST* useInst = use.first;
         Gen4_Operand_Number opndPos = use.second;
-        if (!useInst->isLogic() || !G4_INST::isSrcNum(opndPos))
+        if (!useInst->isLogic() || !G4_INST::isSrcNum(opndPos) ||
+            useInst->getSingleDef(opndPos) == nullptr /* not single def */)
         {
             canFold = false;
             break;
         }
 
+        // sanity check
+        assert(useInst->getSingleDef(opndPos) == notInst);
+
         // check the case where flag is partially used
         G4_SrcRegRegion* opnd = useInst->getSrc(G4_INST::getSrcNum(opndPos))->asSrcRegRegion();
         if (dst->compareOperand(opnd) != Rel_eq)