[X86] Remove incomplete custom handling of i128 sdivrem/udivrem on Windows.

We need to have special handling of i128 div/rem on Windows due
to a weird calling convention needed for the libcall. There was
also some code that made it look like we do the same for sdivrem/udiv,
but the code didn't account for multiple return values of those
functions so couldn't possibly work. I think this code never
triggers because we don't have libcall names defined for those
functions by default so DAGCombine never creates DIVREM nodes.

Author: topperc

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -1929,8 +1929,6 @@ X86TargetLowering::X86TargetLowering(const X86TargetMachine &TM,
     setOperationAction(ISD::UDIV, MVT::i128, Custom);
     setOperationAction(ISD::SREM, MVT::i128, Custom);
     setOperationAction(ISD::UREM, MVT::i128, Custom);
-    setOperationAction(ISD::SDIVREM, MVT::i128, Custom);
-    setOperationAction(ISD::UDIVREM, MVT::i128, Custom);
   }
 
   // On 32 bit MSVC, `fmodf(f32)` is not defined - only `fmod(f64)`
@@ -26899,8 +26897,6 @@ SDValue X86TargetLowering::LowerWin64_i128OP(SDValue Op, SelectionDAG &DAG) cons
   case ISD::UDIV:      isSigned = false; LC = RTLIB::UDIV_I128;    break;
   case ISD::SREM:      isSigned = true;  LC = RTLIB::SREM_I128;    break;
   case ISD::UREM:      isSigned = false; LC = RTLIB::UREM_I128;    break;
-  case ISD::SDIVREM:   isSigned = true;  LC = RTLIB::SDIVREM_I128; break;
-  case ISD::UDIVREM:   isSigned = false; LC = RTLIB::UDIVREM_I128; break;
   }
 
   SDLoc dl(Op);
@@ -29535,10 +29531,6 @@ void X86TargetLowering::ReplaceNodeResults(SDNode *N,
       return;
     }
 
-    LLVM_FALLTHROUGH;
-  }
-  case ISD::SDIVREM:
-  case ISD::UDIVREM: {
     SDValue V = LowerWin64_i128OP(SDValue(N,0), DAG);
     Results.push_back(V);
     return;