spirv: Propagate alignments to deref chains via casts

spirv: Propagate alignments to deref chains via casts

This commit propagates the alignment information provided either through
the Alignment decoration on pointers or via the alignment mem operands
to OpLoad, OpStore, and OpCopyMemory to the NIR deref chain.  It does so
by wrapping the deref in a cast.  NIR should be able to clean up most
unnecessary casts only leaving us with the useful alignment information.

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