message: removing a block must delete position dependencies
  Summary: See title.
diff: @@ -2171,6 +2171,15 @@ void ControlFlowGraph::remove_block(Block* block) {
  }
  delete_pred_edges(block);
  delete_succ_edges(block);
  +
  + std::unordered_set<DexPosition*> deleted_positions;
  + for (const auto& mie : *block) {
  + if (mie.type == MFLOW_POSITION) {
  + deleted_positions.insert(mie.pos.get());
  + }
  + }
  + remove_dangling_parents(deleted_positions);
  +
  auto id = block->id();
  auto num_removed = m_blocks.erase(id);
  always_assert_log(num_removed == 1,
  