Only compare array notations in SQLQuery->getOrderedJoins()

If more than two $from were added through SQLQuery->addFrom(),
the getOrderedJoins() comparison kicks in. It assumes all $from
parts are in array notation, which isn't always the case.
For legacy reasons, and because we don't have full API support,
you can still add literal joins through addFrom('INNER JOIN ...').
On PHP 5.3, the ordering comparison still works because it
allows array access in strings, with string rather than numeric indexes.
Thankfully that's no longer supported in PHP 5.4.