Multiple NestedLoop index joins are not being generated in CBO

Some of these queries produced an internal edge with an assign statement such as $$25 <-- tobigint ($$20)
The assign statement is then used to modify the next join predicate eq($$25, $$30) to
eq(tobigint($$20), $$30).

It turns out that NL index join code only accepts the original join predicate.

The fix will be to push down the assigns from the internal edge to the corresponding leaf node.