Improve TTN so it can be used by Gallium Nine

This MR is part of our ongoing work to support NIR in nine. We are currently testing on radeonsi, but our ultimate goal with this is to make it possible for nine to run with iris (and eventually zink). The nine part is still being worked on.

These patches give some polish to tgsi_to_nir (aka. TTN), so that it can handle the TGSI input that it receives from nine. There are some fixes too so it should also benefit any other users of TTN as well. I've also added a tgsi_to_nir_pipe which takes into account the driver capabilities and produces NIR that is in line with what st_glsl_to_nir outputs. This results in NIR that drivers can actually use (which is not true about what tgsi_to_nir currently outputs).

Any feedback on this patch series is greatly welcome.