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

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>