aco: optimize v_add_u32(v_mul_lo_u16) -> v_mad_u32_u16

aco: optimize v_add_u32(v_mul_lo_u16) -> v_mad_u32_u16

fossils-db (Vega10):
Totals from 779 (0.56% of 139517) affected shaders:
CodeSize: 1187928 -> 1187508 (-0.04%); split: -0.04%, +0.00%
Instrs: 247353 -> 244608 (-1.11%); split: -1.11%, +0.00%
Cycles: 1127472 -> 1116420 (-0.98%); split: -0.98%, +0.00%
VMEM: 139720 -> 138297 (-1.02%); split: +0.00%, -1.02%
SMEM: 51069 -> 50735 (-0.65%); split: +0.04%, -0.69%
Copies: 11548 -> 11547 (-0.01%); split: -0.03%, +0.03%

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7425>