G1CollectedHeap::collection_set_iterate_from() has an unnecessary "if" statement
in which the "then" part can never be executed.

    if (cl->doHeapRegion(cur) && false) {
      cl->incomplete();
      return;
    }

This can be simplified to

  guarantee(!cl->doHeapRegion(cur), err_msg("doHeapRegion returned true"));

Also, G1CollectedHeap::collection_set_iterate() can be simplified because none of the callers
pass a closure where doHeapRegion() returns true.