nir/opt_load_store_vectorize: check for restrict at the variable

nir/opt_load_store_vectorize: check for restrict at the variable

SPIR-V -> NIR doesn't set ACCESS_RESTRICT at the intrinsic.

fossil-db (GFX10.3):
Totals from 3 (0.00% of 139391) affected shaders:
CodeSize: 12364 -> 12356 (-0.06%)
Instrs: 2493 -> 2494 (+0.04%); split: -0.04%, +0.08%
Cycles: 15279372 -> 15295756 (+0.11%); split: -0.11%, +0.21%

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/7295>