[DAGCombiner] Explain why we recreate ADD

Author: pfusik

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -12142,12 +12142,15 @@ SDValue DAGCombiner::visitSELECT(SDNode *N) {
     if (sd_match(Cond1, m_ConstInt(C)) && hasUMin(VT)) {
       if (CC == ISD::SETUGT && Cond0 == N2 &&
           sd_match(N1, m_Add(m_Specific(N2), m_SpecificInt(~C)))) {
+        // The resulting code relies on an unsigned wrap in ADD.
+        // Recreating ADD to drop possible nuw/nsw flags.
         SDValue AddC = DAG.getConstant(~C, DL, VT);
         SDValue Add = DAG.getNode(ISD::ADD, DL, VT, N2, AddC);
         return DAG.getNode(ISD::UMIN, DL, VT, Add, N2);
       }
       if (CC == ISD::SETULT && Cond0 == N1 &&
           sd_match(N2, m_Add(m_Specific(N1), m_SpecificInt(-C)))) {
+        // Ditto.
         SDValue AddC = DAG.getConstant(-C, DL, VT);
         SDValue Add = DAG.getNode(ISD::ADD, DL, VT, N1, AddC);
         return DAG.getNode(ISD::UMIN, DL, VT, N1, Add);