aco: prefer 4-byte aligned definitions

aco: prefer 4-byte aligned definitions

shader-db (Navi, fp16 enabled):
Totals from 42 (0.03% of 127638) affected shaders:
CodeSize: 811984 -> 806224 (-0.71%)
Instrs: 155733 -> 155939 (+0.13%); split: -0.04%, +0.18%
Cycles: 1982568 -> 1984400 (+0.09%); split: -0.06%, +0.15%
VMEM: 7187 -> 7121 (-0.92%); split: +0.86%, -1.78%
SMEM: 1770 -> 1769 (-0.06%)
VClause: 1475 -> 1476 (+0.07%)
Copies: 12406 -> 12606 (+1.61%); split: -0.46%, +2.07%
Branches: 5901 -> 5900 (-0.02%); split: -0.25%, +0.24%

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/5040>