[X86][FastIsel] Teach how to select float-half conversion intrinsics.

This patch teaches X86FastISel how to select intrinsic 'convert_from_fp16' and
intrinsic 'convert_to_fp16'.
If the target has F16C, we can select VCVTPS2PHrr for a float-half conversion,
and VCVTPH2PSrr for a half-float conversion.

Differential Revision: http://reviews.llvm.org/D7673

llvm-svn: 230043