[DAG] visitXOR - add missing comment for or/and constant demorgan fold. NFC.

Noticed while triaging llvm#112347 which is using this fold - we described the or->and fold, but not the equivalent and->or which is also handled.

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -9606,6 +9606,7 @@ SDValue DAGCombiner::visitXOR(SDNode *N) {
     }
   }
   // fold (not (or x, y)) -> (and (not x), (not y)) iff x or y are constants
+  // fold (not (and x, y)) -> (or (not x), (not y)) iff x or y are constants
   if (isAllOnesConstant(N1) && N0.hasOneUse() &&
       (N0Opcode == ISD::OR || N0Opcode == ISD::AND)) {
     SDValue N00 = N0.getOperand(0), N01 = N0.getOperand(1);