nir/lower_io: Apply alignments from derefs when available

nir/lower_io: Apply alignments from derefs when available

If the deref has no explicit alignment in the chain, we assume component
alignment which is what we currently assume for all derefs today.  This
should be correct for all APIs in the sense that we can usually assume
at least component alignment.  However, for some APIs such as OpenCL, we
could potentially make larger alignment assumptions.  The intention is
that those will be handled via alignment-increasing casts.

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>