glsl: Create and use a new ir_variable::count_attribute_slots() wrapper.

This wraps glsl_type::count_attribute_slots(), but will soon contain a
couple of overrides for a couple of GLSL built-ins variables.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>

src/compiler/glsl/ir.h

@@ -553,6 +553,8 @@ public:
       return this->u.max_ifc_array_access;
    }
 
+   unsigned count_attribute_slots(bool is_vertex_stage) const;
+
    inline unsigned get_num_state_slots() const
    {
       assert(!this->is_interface_instance()