MDEV-34665 Simplify IN predicate processing for NULL-aware materialization involving only one column

It was found that unnecessary work of building Ordered_key structures
is being done when processing NULL-aware materialization for IN predicates
having only one column. In fact, the logic for that simplified case can be
expressed as follows.
Say we have predicate left_expr IN (SELECT <subq1>), where left_expr is
scalar(not a tuple).
Then
    if (left_expr is NULL) {
      if (subq1 produced any rows) {
        // note that we don't care if subq1 has produced
        // NULLs or not.
        NULL IN (<some values>) -> UNKNOWN, i.e. NULL.
      } else {
        NULL IN ({empty-set}) -> FALSE.
      }
    } else {
      // left_expr is a non-NULL value
      if (subq1 output has a match for left_expr) {
        left_expr IN (..., left_expr ...) -> TRUE
      } else {
        // no "known" matches.
        if (subq1 output has a NULL) {
          left_expr IN ( ... NULL ...) ->
           (NULL could have been a match or not)
           -> NULL.
        } else {
          // subq1 didn't produce any "UNKNOWNs" so
          // we're positive there weren't any matches
          -> FALSE.
        }
      }
    }

This commit introduces subselect_single_column_partial_engine class
implementing the logic described.