#522 Incorrect filter optimization in Saxon-SA

[Michael Kay]

When the Saxon-SA optimizer decides that a filter
expression applied to a variable, such as

$idElements[@id=$hrefs]

would benefit from construction of an index, the result
of the indexed retrieval is not (always) sorted and
deduplicated, which means that items from the sequence
$idElements can appear more than once and in the wrong
order.

(For my reference: source fix is in
IndexedClosure.findItems)