turnip: Do gathering xfb info after nir_remove_dead_variables

turnip: Do gathering xfb info after nir_remove_dead_variables

So we could align stream outputs correctly even if unused in/outs are
removed.

Fixes:
  dEQP-VK.transform_feedback.fuzz.random_vertex.scalar_types.*
  dEQP-VK.transform_feedback.fuzz.random_vertex.vector_types.*

Signed-off-by: Hyunjun Ko <zzoon@igalia.com>
Reviewed-by: Jonathan Marek <jonathan@marek.ca>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4207>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4207>