nir: do not try to merge xfb-outputs

nir: do not try to merge xfb-outputs

It's tricky to merge XFB-outputs correctly, because we need there to not
be any overlaps when we get to `nir_gather_xfb_info_with_varyings` later
on. We currently trigger an assert there if we end up merging here.

So let's not even try. This is an optimization, and we can optimize this
in safe cases later if needed. For now, let's play it safe.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5329>