move more to tgsi_from_mesa, use from nouveau

Emma Anholt requested to merge anholt/mesa:nouveau-nir into master

As part of my nir-to-tgsi series, I found I wanted to move some of this mesa-to-tgsi code to gallium's new shared location for it, and that nouveau had a separate stash of that code that we could delete

Untested on nouveau, hopefully @karolherbst can do that for me.

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