[AArch64] Handle ISD::LRINT and ISD::LLRINT

This patch optimizes ISD::LRINT and ISD::LLRINT to frintx plus
fcvtzs. It currently only handles the scalar version.

Reviewed By: SjoerdMeijer, mstorsjo

Differential Revision: https://reviews.llvm.org/D62018

llvm-svn: 361877