nir/opt_deref: Remove restrictive alignment information from casts

nir/opt_deref: Remove restrictive alignment information from casts

If we have a cast deref with alignment information and we can get equal
or better alignment information from something further up the deref
chain, we can strip the alignment information from the cast and allow
other optimizations to potentially eliminate the cast.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6472>