RFR(S): 8212611: Small collection of simple changes from shenandoah

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8212611/webrev.00/

- Signature of enqueue_useful_gc_barrier(),
  eliminate_useless_gc_barriers() is modified.

- Call set_req_X() in some places so disconnected input has a chance to
 be reprocessed

- Add calls to step_over_gc_barrier() in places where peeking through a
  barrier helps local optimizations.

In isa_java_mirror_load(), I dropped the call is_gc_barrier_node()
because for zgc (the only gc for which it is implemented), it duplicates
the logic of step_over_gc_barrier() so it's more efficient to simply
call step_over_gc_barrier().

Roland.