message: Fix `remove_empty_blocks`
  Summary: Gotta make forward progress in the iterator.
diff: @@ -1084,6 +1084,7 @@ void ControlFlowGraph::remove_empty_blocks() {
  } else if (g_redex->instrument_mode) {
  // If we are instrumenting, it is necessary to keep the block for its
  // source-blocks.
  + ++it;
  continue;
  }
  }
  