nir/lower_goto_if: Rework some set union logic

I find the ternary a bit hard to read.  The optimization is fairly
obvious but the way it's coded makes things more dense than they
probably need to be.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2401>
1 file changed