nir/builder: Cast array indices in build_deref_follower

nir/builder: Cast array indices in build_deref_follower

There's no guarantee when build_deref_follower is called that the two
derefs have the same bit size destination.  Insert a cast on the array
index in case we have differing bit sizes.  While we're here, insert
some asserts in build_deref_array and build_deref_ptr_as_array.  The
validator will catch violations here but they're easier to debug if we
catch them while building.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>