Add ISD::FROUND for libm round()

All libm floating-point rounding functions, except for round(), had their own
ISD nodes. Recent PowerPC cores have an instruction for round(), and so here I'm
adding ISD::FROUND so that round() can be custom lowered as well.

For the most part, this is straightforward. I've added an intrinsic
and a matching ISD node just like those for nearbyint() and friends. The
SelectionDAG pattern I've named frnd (because ISD::FP_ROUND has already claimed
fround).

This will be used by the PowerPC backend in a follow-up commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187926 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/BasicTargetTransformInfo.cpp

@@ -449,6 +449,7 @@ unsigned BasicTTI::getIntrinsicInstrCost(Intrinsic::ID IID, Type *RetTy,
   case Intrinsic::nearbyint:
                            ISD = ISD::FNEARBYINT; break;
   case Intrinsic::rint:    ISD = ISD::FRINT;  break;
+  case Intrinsic::round:   ISD = ISD::FROUND; break;
   case Intrinsic::pow:     ISD = ISD::FPOW;   break;
   case Intrinsic::fma:     ISD = ISD::FMA;    break;
   case Intrinsic::fmuladd: ISD = ISD::FMA;    break; // FIXME: mul + add?