nir: rename nir_link_constant_varyings() nir_link_opt_varyings()

The following patches will add support for an additional
optimisation so this function will no longer just optimise varying
constants.

Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/intel/compiler/brw_nir.c

@@ -771,7 +771,7 @@ brw_nir_link_shaders(const struct brw_compiler *compiler,
       *consumer = brw_nir_optimize(*consumer, compiler, c_is_scalar, false);
    }
 
-   if (nir_link_constant_varyings(*producer, *consumer))
+   if (nir_link_opt_varyings(*producer, *consumer))
       *consumer = brw_nir_optimize(*consumer, compiler, c_is_scalar, false);
 
    NIR_PASS_V(*producer, nir_remove_dead_variables, nir_var_shader_out);