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/SelectionDAG/LegalizeVectorOps.cpp

@@ -241,6 +241,7 @@ SDValue VectorLegalizer::LegalizeOp(SDValue Op) {
   case ISD::FTRUNC:
   case ISD::FRINT:
   case ISD::FNEARBYINT:
+  case ISD::FROUND:
   case ISD::FFLOOR:
   case ISD::FP_ROUND:
   case ISD::FP_EXTEND: