Predicate getPostUpdateReverseStep

Holds if a reverse local flow step should be added from the post-update node for e to the post-update node for the result.

This is needed to allow for side-effects on compound expressions to propagate to sub components. For example, in

(foo1; foo2).set_field(taint)

we add a reverse flow step from [post] (foo1; foo2) to [post] foo2, in order for the side-effect of set_field to reach foo2.