[Mesa-dev] Initial GS NIR support for radeonsi

[Timothy Arceri]

The support is still WIP but the patches as starting to pile
up so thought I'd see if I could land these before continuing.

Whats missing?

Vega support for gs_vtx_offset handling (see patch 3), I don't
have one yet for testing so didn't attempt to adapt the code.

Lots of piglit tests still fail. The gs clip distance piglit
tests currently cause my system to hang for instance.