During startup, before `is_init_completed` returns true, GC can't occur.

For example, in `VM_GC_Operation::doit_prologue`:

  if (!is_init_completed()) {
    vm_exit_during_initialization(
      err_msg("GC triggered before VM initialization completed. Try increasing "
              "NewSize, current value %zu%s.",
              byte_size_in_proper_unit(NewSize),
              proper_unit_for_byte_size(NewSize)));
  }

To better handle the scenario of the initial heap being too small or too much allocation during startup, one can expand heap depending on `is_init_completed`.