aco: fix waves calculation for wave32

aco: fix waves calculation for wave32

fossil-db (GFX10.3, wave32):
Totals from 176 (0.13% of 139391) affected shaders:
SGPRs: 16648 -> 16640 (-0.05%)
VGPRs: 18920 -> 19076 (+0.82%); split: -0.30%, +1.12%
CodeSize: 2354172 -> 2354288 (+0.00%); split: -0.01%, +0.01%
MaxWaves: 1618 -> 1627 (+0.56%); split: +0.68%, -0.12%
Instrs: 435756 -> 435761 (+0.00%); split: -0.02%, +0.02%
Cycles: 8858360 -> 8869960 (+0.13%); split: -0.01%, +0.14%
VMEM: 55899 -> 57220 (+2.36%); split: +2.53%, -0.17%
SMEM: 10323 -> 10374 (+0.49%); split: +0.73%, -0.23%
VClause: 8307 -> 8290 (-0.20%); split: -0.24%, +0.04%
SClause: 16573 -> 16577 (+0.02%); split: -0.01%, +0.03%
Copies: 24641 -> 24652 (+0.04%); split: -0.24%, +0.28%

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/8761>