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.
Edited by Erik Faye-Lund