Use emit_predicate_on_sample_mask() helper that does check where to
get the correct mask depending on whether discard/demote was used or
not.
Fixes: 45f5db5a84 ("intel/fs: Implement "demote to helper invocation"")
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15400>