Add the nearbyint -> FNEARBYINT mapping to BasicTargetTransformInfo

This fixes an oversight that Intrinsic::nearbyint was not being mapped to
ISD::FNEARBYINT (thus fixing the over-optimistic cost we were assigning to
nearbyint calls for some targets).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185783 91177308-0d34-0410-b5e6-96231b3b80d8