[Mesa-dev] i965 NIR linking

[Timothy Arceri]

This started out based off the work Jason did back in 2015 to add
NIR linking to the Intel VK driver. It needed a reasonable amount
of updates to work with the GL driver, tess, xfb, etc.

As per the results in patch 8, it can provide some nice
improvements despite the GLSL IR linker already doing the same
link time removal of unused varyings.

Ultimately I'd like to use this with radv but adding it to i965
first provides a good test platform given the mature test suites,
and extensive shader-db collections available for OpenGL. I'm
planning on also adding a NIR packing pass and it makes sense
to test that here also. I beleive the packing pass should be the
last set towards removing any dependency on the GLSL IR
optimisation passes.

Please review.