nir: add callback to nir_remove_dead_variables()

nir: add callback to nir_remove_dead_variables()

This allows us to do API specific checks before removing variable
without filling nir_remove_dead_variables() with API specific code.

In the following patches we will use this to support the removal
of dead uniforms in GLSL.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4797>