Fixed #19837 -- Refactored split_exclude() join generation

The refactoring mainly concentrates on making sure the inner and outer
query agree about the split position. The split position is where the
multijoin happens, and thus the split position also determines the
columns used in the "WHERE col1 IN (SELECT col2 from ...)" condition.

This commit fixes a regression caused by #10790 and commit
69597e5bcc89aadafd1b76abf7efab30ee0b8b1a. The regression was caused
by wrong cols in the split position.