Improve create_unique_path to not be fooled by unrelated clauses that happen

Improve create_unique_path to not be fooled by unrelated clauses that happen
to be syntactically part of a semijoin clause.  For example given
WHERE EXISTS(SELECT ... WHERE upper.var = lower.var AND some-condition)
where some-condition is just a restriction on the lower relation, we can
use unique-ification on lower.var after having applied some-condition within
the scan on lower.