i965: Use nir_lower_passthrough_edgeflags

i965: Use nir_lower_passthrough_edgeflags

Now that there's a common NIR pass, there's no point in us doing this in
the back-end anymore.  In order to use this pass in i965, we do have to
make one tiny change.  Gallium runs the pass after assigning input and
output locations and so needs the pass to respect those locations and
num_inputs.  i965, however, runs it before any location assignment or
I/O lowering so we don't care.  We do, however, need the pass to succeed
with num_inputs == 0 because we set that later.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11313>