[SelectionDAG] Rename and clarify visitFMULForFMADCombine (NFC)

[nhaehnle]

Summary

Suggested by @spatel in D26602.

[spatel]

Thanks! LGTM.

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

private:
     SDValue visitMSTORE(SDNode *N);
     SDValue visitMGATHER(SDNode *N);
     SDValue visitMSCATTER(SDNode *N);
     SDValue visitFP_TO_FP16(SDNode *N);
     SDValue visitFP16_TO_FP(SDNode *N);
 
     SDValue visitFADDForFMACombine(SDNode *N);
     SDValue visitFSUBForFMACombine(SDNode *N);
-    SDValue visitFMULForFMACombine(SDNode *N);
+    SDValue visitFMULForFMADistributiveCombine(SDNode *N);
 
     SDValue XformToShuffleWithZero(SDNode *N);
     SDValue ReassociateOps(unsigned Opc, const SDLoc &DL, SDValue LHS,
                            SDValue RHS);
 
     SDValue visitShiftByConstant(SDNode *N, ConstantSDNode *Amt);
 
     SDValue foldSelectOfConstants(SDNode *N);
if (AllowFusion && LookThroughFPExt) {
         }
       }
     }
   }
 
   return SDValue();
 }
 
-/// Try to perform FMA combining on a given FMUL node.
-SDValue DAGCombiner::visitFMULForFMACombine(SDNode *N) {
+/// Try to perform FMA combining on a given FMUL node based on the distributive
+/// law x * (y + 1) = x * y + x and variants thereof (commuted versions,
+/// subtraction instead of addition).
+SDValue DAGCombiner::visitFMULForFMADistributiveCombine(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   SDValue N1 = N->getOperand(1);
   EVT VT = N->getValueType(0);
   SDLoc SL(N);
 
   assert(N->getOpcode() == ISD::FMUL && "Expected FMUL Operation");
 
   const TargetOptions &Options = DAG.getTarget().Options;
if (char RHSNeg = isNegatibleForFree(N1, LegalOperations, TLI, &Options)) {
         return DAG.getNode(ISD::FMUL, DL, VT,
                            GetNegatedExpression(N0, DAG, LegalOperations),
                            GetNegatedExpression(N1, DAG, LegalOperations),
                            Flags);
     }
   }
 
   // FMUL -> FMA combines:
-  if (SDValue Fused = visitFMULForFMACombine(N)) {
+  if (SDValue Fused = visitFMULForFMADistributiveCombine(N)) {
     AddToWorklist(Fused.getNode());
     return Fused;
   }
 
   return SDValue();
 }
 
 SDValue DAGCombiner::visitFMA(SDNode *N) {