spirv: Pull offset/stride from the pointer for OpArrayLength

We can't pull it from the variable type because it might be an array of
blocks and not just the one block.  While we're here, throw in some
error checking.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit f1dbc7e9)