Don't simplify uncorrelated EXISTS sublink in some cases

This is another way to fix #11849, more details can be found at #13823.

And is this fix, there has no regression on uncorrelated EXISTS sublinks that have DISTINCT/DISTINCT ON/ORDER BY clause, but the logic of the function simplify_EXISTS_query() is more complicated.

This PR and #13823 both can fix issue #11849, and #13823 makes simplify_EXISTS_query() easier to understand, but has regression in some cases. This PR makes simplify_EXISTS_query() a little more complicated, but has no regression.