GenBCode: test removal dangling exception handlers

An asm.tree.MethodNode includes a list of exception-handler entries,
each such entry includes (among the other things) start and end
LabelNodes to demarcate the range being protected.

  - Any executable instruction other than NOP means the
    exception-handler-entry should stay.

  - Otherwise, asm.optimiz.DanglingExcHandlers removes
    the exception-handler entry in question.