nir: Tag entrypoint for easy recognition by nir_shader_get_entrypoint()

We're going to have multiple functions, so nir_shader_get_entrypoint()
needs to do something a little smarter.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir_clone.c

@@ -684,6 +684,7 @@ clone_function(clone_state *state, const nir_function *fxn, nir_shader *ns)
    nfxn->num_params = fxn->num_params;
    nfxn->params = ralloc_array(state->ns, nir_parameter, fxn->num_params);
    memcpy(nfxn->params, fxn->params, sizeof(nir_parameter) * fxn->num_params);
+   nfxn->is_entrypoint = fxn->is_entrypoint;
 
    /* At first glance, it looks like we should clone the function_impl here.
     * However, call instructions need to be able to reference at least the