i965: Rework the TCS passthrough shader to use NIR.

i965: Rework the TCS passthrough shader to use NIR.

I'm about to implement a scalar TCS backend, and I'd rather not
duplicate all of this code there.

One change is that we now write the tessellation levels from all
TCS threads, rather than just the first.  This is pretty harmless,
and was easier.  The IF/ENDIF needed for that are gone; otherwise
the generated code is basically identical.

I chose to emit load/store intrinsics directly because it was easier.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>