nir/schedule: Add a callback for backend-specific dependencies

nir/schedule: Add a callback for backend-specific dependencies

Adds a callback function to nir_schedule_options to give the backend a
chance to add custom dependencies between certain intrinsics. The
callback can assign a class number to the intrinsic and then set a read
or write dependency on that class.

v2: Use a linked-list of schedule nodes for the dependency classes
    instead of a fixed-sized array.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5953>