i965: Add opcodes for F32TO16 and F16TO32

i965: Add opcodes for F32TO16 and F16TO32

The GLSL ES 3.00 operations packHalf2x16 and unpackHalf2x16 will emit
these opcodes.

- Define the opcodes BRW_OPCODE_{F32TO16,F16TO32}.
- Add the opcodes to the brw_disasm table.
- Define convenience functions brw_{F32TO16,F16TO32}.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Acked-by: Paul Berry <stereotype441@gmail.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>