aco: only use SMEM if we can prove it's safe

aco: only use SMEM if we can prove it's safe

Totals from 26 (0.02% of 127638) affected shaders:
SGPRs: 1680 -> 1664 (-0.95%)
VGPRs: 1492 -> 1504 (+0.80%)
CodeSize: 233140 -> 233016 (-0.05%); split: -0.09%, +0.04%
Instrs: 47121 -> 47114 (-0.01%); split: -0.08%, +0.06%
VMEM: 4930 -> 4655 (-5.58%); split: +0.12%, -5.70%
SMEM: 2030 -> 2001 (-1.43%); split: +3.79%, -5.22%
VClause: 891 -> 947 (+6.29%)
SClause: 876 -> 816 (-6.85%)
Copies: 4734 -> 4716 (-0.38%); split: -0.40%, +0.02%
Branches: 2048 -> 2047 (-0.05%)
PreSGPRs: 1400 -> 1396 (-0.29%)
PreVGPRs: 1440 -> 1443 (+0.21%)

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/5207>