nir/opt_shrink_vectors: shrink image stores using the format

nir/opt_shrink_vectors: shrink image stores using the format

fossil-db (Navi):
Totals from 657 (0.48% of 135946) affected shaders:
VGPRs: 26076 -> 25520 (-2.13%); split: -2.15%, +0.02%
CodeSize: 3033016 -> 3014472 (-0.61%); split: -0.64%, +0.03%
MaxWaves: 9386 -> 9420 (+0.36%)
Instrs: 590109 -> 585502 (-0.78%); split: -0.82%, +0.04%

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5772>