i965/fs: Implement FS_OPCODE_[UN]PACK_HALF_2x16_SPLIT[_XY] opcodes.

i965/fs: Implement FS_OPCODE_[UN]PACK_HALF_2x16_SPLIT[_XY] opcodes.

I'd neglected to port these to Broadwell.  Most of this code is copy
and pasted from Gen7, but instead of using F32TO16/F16TO32, we just
use MOV with HF register types.

Fixes fs-packHalf2x16 and fs-unpackHalf2x16 tests (both the ARB
extension and ES 3.0 variants).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>