aco: use s_pack_ll_b32_b16 for constant copies

Totals from 2 (0.00% of 134913) affected shaders:
CodeSize: 28636 -> 28628 (-0.03%)

Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20970>
1 file changed