Transform

in (select inner_col' from inner_table where inner_col = outer_col)

to

, outer_col in (select inner_col', inner_col from inner_table)

Achieved by implementing Item_in_subselect::exists2in_processor(),
accompanied with comprehensive test coverage.

Caveat:

- The fix of 2nd ps execution segfault requires
  HAVE_PSI_STATEMENT_INTERFACE because it checks
  thd->m_statement_state.m_parent_prepared_stmt to determine whether we
  are inside a ps execution. Is there a better way to determine this?
- Cannot recognise bad item mismatch in equalities that causes
  materialization to not materialize down the road