need explanation for some code in DAGCombiner::useDivRem

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I have a target for which UDIV/SDIV is legal, UMOD/SMOD requires an instruction prefix a divide instruction, and UDIVMOD/SDIVMOD is the same as UMOD/SMOD.

In DAGCombiner::useDivRem there is this code:

   // If div is legal, it's better to do the normal expansion
   unsigned OtherOpcode = 0;
   if ((Opcode == ISD::SDIV) || (Opcode == ISD::UDIV)) {
     OtherOpcode = isSigned ? ISD::SREM : ISD::UREM;
     if (TLI.isOperationLegalOrCustom(Opcode, VT))
       return SDValue();
   } else {
     OtherOpcode = isSigned ? ISD::SDIV : ISD::UDIV;
     if (TLI.isOperationLegalOrCustom(OtherOpcode, VT))
       return SDValue();
   }

This prevents generation of UDIVMOD/SDIVMOD because UDIV/SDIV is legal.
Why is this check made? An what does "it's better to do the normal expansion" mean?

Thanks,
brian