The idea behind this change is, that AlterUniqueTogether,
AlterIndexTogether and AlterOrderWithRespectTo can always be moved after
an Add/Alter/Rename/RemoveField operation if they don't refer to the
respective field and are not empty sets / None.
Combined with the optimizations of duplicate AlterUniqueTogether,
AlterIndexTogether, and AlterOrderWithRespectTo operations from
128caa1e16ec2627737748f75c8e55600a3df97f, these operations are optimized
in a later round of the optimizer.
Thanks Tim Graham for the review.