glsl: implement lower_xfb_varying() as a NIR pass

This just converts the GLSL IR pass to NIR.

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

src/compiler/glsl/meson.build

@@ -89,6 +89,7 @@ files_libglsl = files(
   'gl_nir_lower_packed_varyings.c',
   'gl_nir_lower_samplers.c',
   'gl_nir_lower_samplers_as_deref.c',
+  'gl_nir_lower_xfb_varying.c',
   'gl_nir_link_atomics.c',
   'gl_nir_link_uniform_blocks.c',
   'gl_nir_link_uniform_initializers.c',