aco: improve nir_op_vec with constant operands

aco: improve nir_op_vec with constant operands

Could still be improved a little. For example, 8-bit pack without
constants could be:
(s_pack_ll(x, z) & 0x00ff00ff) | ((s_pack_ll(y, w) & 0x00ff00ff) << 8)

fossil-db (Sienna):
Totals from 136 (0.10% of 139391) affected shaders:
CodeSize: 279776 -> 278144 (-0.58%)
Instrs: 50742 -> 50470 (-0.54%)
Cycles: 211560 -> 210472 (-0.51%)
SMEM: 3607 -> 3557 (-1.39%)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8421>