r600/sfn: rewrite NIR backend
This is a rewrite of the NIR backend, it adds a backend optimizer and a scheduler.
There are still a few regressions with respect to the NTT / TGSI backend, but also a number of fixes. Especially because of a big regression when running Xonotic sb is still enabled by default.
Compared to the old backend it provides a better performance, and hardware fp64 is actually fp64 handled on Cayman.
The backend supports Evergreen based cards and up, and it was tested on CEDAR and CAYMAN.