Partial subgroups can lead to infinite loops otherwise.
cc: mesa-stable
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27603>
(cherry picked from commit 4d7beb22fae3fe10aed86066ee9d2d9536625a72)