compiler/nir_types: Handle vectors in glsl_get_array_element

compiler/nir_types: Handle vectors in glsl_get_array_element

Most of NIR doesn't allow doing array indexing on a vector (though it
does on a matrix).  However, nir_lower_io handles it just fine and this
behavior is needed for shared variables in Vulkan.  This commit makes
glsl_get_array_element do something sensible for vector types and makes
nir_validate happy with them.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>