glsl: move common link time optimisation calls to linker code

In the following patch we will move the users of this function to
this file too and make it static again.

Acked-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16770>

src/compiler/glsl/gl_nir_linker.h

@@ -54,6 +54,8 @@ struct gl_nir_linker_options {
 
 void gl_nir_opts(nir_shader *nir);
 
+void gl_nir_link_opts(nir_shader *producer, nir_shader *consumer);
+
 bool gl_nir_link_spirv(const struct gl_constants *consts,
                        struct gl_shader_program *prog,
                        const struct gl_nir_linker_options *options);