glsl: add can_remove_uniform() helper to the NIR linker

glsl: add can_remove_uniform() helper to the NIR linker

This helper reflects the rules we follow in the GLSL IR linker when
deciding if we can remove a dead uniform. This check is required to
avoid regressions when turning on NIR dead uniform clean up in the
following patch.

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>