aco: use nir_addition_might_overflow to combine additions into SMEM

aco: use nir_addition_might_overflow to combine additions into SMEM

fossil-db (Navi):
Totals from 24656 (18.14% of 135946) affected shaders:
CodeSize: 120077160 -> 118877304 (-1.00%); split: -1.01%, +0.01%
Instrs: 23192657 -> 22979553 (-0.92%); split: -0.94%, +0.02%
VMEM: 165151115 -> 151861460 (-8.05%); split: +0.14%, -8.19%
SMEM: 18133265 -> 16709635 (-7.85%); split: +0.28%, -8.13%
VClause: 385011 -> 384447 (-0.15%); split: -0.16%, +0.02%
SClause: 954884 -> 838266 (-12.21%); split: -12.34%, +0.12%

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/2720>