The post-nir_register future

FYI for people who don't follow GitLab--

We've now removed the following deprecated functionality from NIR:

* nir_register
* ALU write masks
* abs/neg/sat modifiers

For background on how we did it and why, see the description of
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23089 and the
transition tracker
https://gitlab.freedesktop.org/mesa/mesa/-/issues/9051

Now that this is merged, casual benchmarking shows ~8% speed-up in
shader compile time.

Going forward, there's a lot of clean up potential. The plan is outlined
in https://gitlab.freedesktop.org/mesa/mesa/-/issues/9396 ... I am
holding off on this until NVK is merged for Faith's rebasing benefit.

If you are sitting on big NIR branches, now is the time to land them.

Here's to the new NIR :~)

Cheers,

-A