nir: add varying component packing helpers

v2: update shader info input/output masks when pack components
v3: make sure interpolation loc matches, this is required for the
    radeonsi NIR backend.
v4: 33dca36f4f fixed nir_gather_info to update outputs_read
    correct, make sure we also adjust this correctly when
    packing components.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl> (v1)
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com> (v3)

src/compiler/nir/nir.h

@@ -2459,6 +2459,8 @@ void nir_assign_var_locations(struct exec_list *var_list, unsigned *size,
 
 /* Some helpers to do very simple linking */
 bool nir_remove_unused_varyings(nir_shader *producer, nir_shader *consumer);
+void nir_compact_varyings(nir_shader *producer, nir_shader *consumer,
+                          bool default_to_smooth_interp);
 
 typedef enum {
    /* If set, this forces all non-flat fragment shader inputs to be