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>

src/compiler/nir_types.cpp

@@ -39,6 +39,8 @@ glsl_get_array_element(const glsl_type* type)
 {
    if (type->is_matrix())
       return type->column_type();
+   else if (type->is_vector())
+      return type->get_scalar_type();
    return type->fields.array;
 }