aco: add emit_mimg() helper

aco: add emit_mimg() helper

Some fossil-db noise from slightly different order of instructions.

fossil-db (GFX10.3):
Totals from 73 (0.05% of 139391) affected shaders:
SGPRs: 3424 -> 3440 (+0.47%)
CodeSize: 199076 -> 199064 (-0.01%); split: -0.01%, +0.00%
Instrs: 37303 -> 37300 (-0.01%); split: -0.01%, +0.00%
Cycles: 786328 -> 786316 (-0.00%); split: -0.00%, +0.00%
VMEM: 19448 -> 19454 (+0.03%); split: +0.04%, -0.01%
SMEM: 5241 -> 5305 (+1.22%); split: +1.70%, -0.48%
SClause: 1282 -> 1281 (-0.08%)

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/8523>