aco: simplify gathering of MIMG address components

aco: simplify gathering of MIMG address components

This patch has a slight effect on pipelinedb:
Totals from affected shaders:
SGPRS: 23616 -> 21504 (-8.94 %)
VGPRS: 15088 -> 14444 (-4.27 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 662660 -> 664600 (0.29 %) bytes
LDS: 49 -> 49 (0.00 %) blocks
Max Waves: 3079 -> 3204 (4.06 %)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3602>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3602>