aco: allow overflow for some SMEM instructions

aco: allow overflow for some SMEM instructions

fossil-db (Navi):
Totals from 10184 (7.49% of 135946) affected shaders:
CodeSize: 83419748 -> 82430824 (-1.19%); split: -1.19%, +0.01%
Instrs: 16054612 -> 15908523 (-0.91%); split: -0.93%, +0.02%
VMEM: 1608018 -> 1581829 (-1.63%); split: +0.20%, -1.83%
SMEM: 577031 -> 563492 (-2.35%); split: +0.10%, -2.45%
VClause: 242643 -> 242512 (-0.05%); split: -0.06%, +0.00%
SClause: 640966 -> 569897 (-11.09%)

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>