nir: Assert memory loads are aligned

nir: Assert memory loads are aligned

We've had alignment parameters on these operations for a while but a
bunch of places weren't setting them.  That should be resolved now so we
can start validating that they're always set.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4441>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4441>