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.c

@@ -208,6 +208,7 @@ nir_function_create(nir_shader *shader, const char *name)
    func->num_params = 0;
    func->params = NULL;
    func->impl = NULL;
+   func->is_entrypoint = false;
 
    return func;
 }