[CodeGen] Add lrint/llrint builtins

This patch add the ISD::LRINT and ISD::LLRINT along with new
intrinsics.  The changes are straightforward as for other
floating-point rounding functions, with just some adjustments
required to handle the return value being an interger.

The idea is to optimize lrint/llrint generation for AArch64
in a subsequent patch.  Current semantic is just route it to libm
symbol.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D62017

llvm-svn: 361875