After trying multiple inheritance (to messy and hard make it work) and
  sublassing jump_if_not (worked, but ugly), decided to on this solution
  instead:
  Inserting an abstract sp_instr_opt_meta class as parent for all instructions
  with destinations makes it possible to handle a continuation pointer for
  sp_instr_set_case_expr too.
  Note: No special test case; the fix is captured by the changed behaviour of
  bug14643_2, and bug14498_4 (formerly disabled), in sp.test.