Item_subselect::is_expensive() used to return FALSE (Inexpensive) whenever
it saw that one of SELECTs in the Subquery's UNION is degenerate. It
ignored the fact that other parts of the UNION might not be inexpensive,
including the case where pther parts of the UNION have no query plan yet.
For a subquery in form col >= ANY (SELECT 'foo' UNION SELECT 'bar')
this would cause the query to be considered inexpensive when there is
no query plan for the second part of the UNION, which in turn would
cause the SELECT 'foo' to compute and free itself while still inside
JOIN::optimize for that SELECT (See MDEV comment for full description).
If the optimizer switch 'semijoin_with_cache' is set to 'off' then
join cache cannot be used to join inner tables of a semijoin.
Also fixed a bug in the function check_join_cache_usage() that led
to wrong output of the EXPLAIN commands for some test cases.
