message: add help functions in cfg iterator
  Summary: as title
diff: @@ -1429,6 +1429,18 @@ class InstructionIteratorImpl {
  m_it == ir_list::InstructionIteratorImpl<is_const>();
  }
  + // \returns true if current iterator is at the end of current block.
  + bool is_end_in_block() const {
  + return m_it.unwrap() == m_block->second->m_entries.end();
  + }
  +
  + // Move the current iterator and this move must be in the same block.
  + InstructionIteratorImpl<is_const>& move_next_in_block() {
  + always_assert(!is_end_in_block());
  + ++m_it;
  + return *this;
  + }
  +
  Iterator unwrap() const { return m_it.unwrap(); }
  Block* block() const {
  