RFR: Factor out keep-alive barrier

[Aleksey Shipilev]

On 09/06/2017 01:44 PM, Roman Kennke wrote:
> http://cr.openjdk.java.net/~rkennke/keepalivebarrier/webrev.00/

 *) This does not look correct with compressed references, used to be "oop obj = *p", because the
argument is oop*, not HeapWord*:

  448 void Klass::klass_update_barrier_set_pre(oop* p, oop v) {
  449   oop obj = oopDesc::load_heap_oop(p);

 *) It is a bit worrying to remove the INCLUDE_ALL_GC blocks from the hotpaths in e.g. jni.cpp

 *) ShenandoahSATBBarrier should be handled in ShenandoahBarrierSet::keep_alive_barrier?

-Aleksey