ASSERT FAIL when code generating a column reference in a join predicate in presence of other outer join reordering

From schema given in DERBY-4712, this query gives an ASSERT with sane Derby:

rs = s.executeQuery("SELECT 1 FROM (T0 LEFT JOIN (T1 LEFT JOIN (T2 LEFT JOIN " +
" (T3 LEFT JOIN T4 ON 1=1) ON T2.X = T3.X) ON 1=1) ON 1=1) " +
" LEFT JOIN " +
" (T5 INNER JOIN T6 ON 1=1) " +
" ON T2.X = 1 ");

Cf the attachments in DERBY-4712 assert-bind-opt-trees.*.
From preliminary analysis, this error seems to be unrelated to the NPEs reported in DERBY-4712, so filing this as a sub-issue.