nir/load_store_vectorize: rework alignment calculation

nir/load_store_vectorize: rework alignment calculation

It now also updates align_offset and creates better alignment information
with a constant 0 offset.

shader-db (Navi):
Totals from 63 (0.05% of 127638) affected shaders:
SGPRs: 3072 -> 3064 (-0.26%)
VGPRs: 2736 -> 2740 (+0.15%)
CodeSize: 325180 -> 324336 (-0.26%); split: -0.27%, +0.01%
Instrs: 63555 -> 63413 (-0.22%); split: -0.24%, +0.02%

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/4710>