RFR: Fix ReferenceProcessor related assert

[Roman Kennke]

Aleksey recently found an assert:

#  Internal Error (/opt/jenkins/workspace/jdk9-shenandoah-
fastdebug/hotspot/src/share/vm/gc/shenandoah/shenandoahBarrierSet.cpp:2
72), pid=29026, tid=29028
#  assert(o == __null || oopDesc::unsafe_equals(o,
resolve_oop_static(o))) failed: only write to-space values

coming from:

V  [libjvm.so+0xa4e87a]  report_vm_error(char const*, int, char const*,
char const*, ...)+0xea
V  [libjvm.so+0x13b374f]  ShenandoahBarrierSet::write_ref_field_work(vo
id*, oop, bool)+0x11f
V  [libjvm.so+0x132f0e7]  BarrierSet::write_ref_field(void*, oop,
bool)+0x57
V  [libjvm.so+0x132c6dd]  ReferenceProcessor::enqueue_discovered_reflis
t(DiscoveredList&)+0x71d

I think this is harmless, but needs some treatment. What happens is
this: in enqueue_discovered_reflist() it calls
swap_reference_pending_list() which can give us a from-space reference
(GC roots in Universe get updated after weakref processing!). Then it
stores that in set_discovered_raw() which is ok, because that does the
correct read-barrier before storing, but then goes on to call
write_ref_field() which, for Shenandoah, only does assert a few things.
And blows up when it gets a from-space reference.

The cheapest fix is to do a read-barrier in debug build.

http://cr.openjdk.java.net/~rkennke/fixrefproc/webrev.00/

Ok to push?

Roman