glsl: add a NIR based varying linker

With a NIR based linker we get better xfb packing, and we no longer
depend on the GLSL IR optimisations to be able to link shaders
with a large amount of dead input/outputs.

Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15731>

src/compiler/glsl/meson.build

@@ -93,6 +93,8 @@ files_libglsl = files(
   'gl_nir_link_uniform_blocks.c',
   'gl_nir_link_uniform_initializers.c',
   'gl_nir_link_uniforms.c',
+  'gl_nir_link_varyings.c',
+  'gl_nir_link_varyings.h',
   'gl_nir_link_xfb.c',
   'gl_nir_linker.c',
   'gl_nir_linker.h',