i965: Drop bogus F32TO16/F16TO32 instructions on Broadwell - use MOV.

i965: Drop bogus F32TO16/F16TO32 instructions on Broadwell - use MOV.

Broadwell removed the F32TO16 and F16TO32 instructions.  However, it has
actual support for HF values, so they're actually just MOV.

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

v2: Emulate F32TO16's align16 zeroing bug, since Chad's front end code
    relies on it happening.  We can probably refactor this code to be
    better later.

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