nir: Remove old-school deref chain support

Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir_split_var_copies.c

@@ -123,8 +123,6 @@ nir_split_var_copies(nir_shader *shader)
 {
    bool progress = false;
 
-   nir_assert_unlowered_derefs(shader, nir_lower_load_store_derefs);
-
    nir_foreach_function(function, shader) {
       if (function->impl)
          progress = split_var_copies_impl(function->impl) || progress;