is
Answer whether this plan-in-progress is at a backward jump instruction.
An A_BundleTree is expanded and kept in memory during parsing of a module. When a method with a large number of repetitions is parsed, this would normally cause the bundle tree to expand and occupy more memory than is really needed. Instead, we notice when a bundle tree node has reached the same plan-in-progress state as one of its ancestor bundle tree nodes, and simply link it back to the existing node, saving the memory space and the time to expand the bundle 'tree' repeatedly.
Return
Whether it jumps backward from here.