ac/nir: implement missing nir_op_pack_half_2x16_split

RADV now lowers lower_pack_half_2x16.

Fixes: 2f125908b35 ("radv,aco: lower_pack_half_2x16")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7187>
1 file changed