Fixes a scenario where an IN subquery returned the wrong result
because the pushed WHERE clause was not retained for downstream
result filtering. For example:
CREATE TABLE t1 (c1 TEXT, UNIQUE (c1(1)));
INSERT INTO t1 (c1) VALUES ('a');
SELECT 'abc' IN (SELECT c1 FROM t1);
Internally, he 'abc' IN subquery condition becomes the constant
condition:
'abc' = t1.c1 or t1.c1 is null
Prior to this patch, this condition was incorrectly removed when
converting the subquery engine to an index lookup-based engine.
Now eligible conditions are preserved during such engine rewrites.
