MDEV-28127 did is_equal() which compared vcol expressions
literally. But another table vcol expression is not equal because of
different table name.

We implement another comparison method is_identical() which respects
different table name in vcol comparison. If any field item points to
table_A and compared field item points to table_B, such items are
treated as equal in (table_A, table_B) comparison. This is done by
cloning table_B expression and renaming any table_B entries to table_A
in it.