RFR: 8075215: SATB buffer processing found reclaimed humongous object

[Kim Barrett]

Please review this fix for a problem with eager reclaim of humongous
objects.

The elements of SATB buffers are being processed in the same way as
elements of the mark queue and when scanning objects (e.g. by calling
CMTask::deal_with_reference).  That no longer works because SATB
buffers may contain references to reclaimed humongous objects.

Scrubbing the SATB buffers after eager reclaim of humongous objects
would extend the pause.  We instead take advantage of the tests needed
for deciding whether a reference needs to be marked being sufficient
to also filter out stale references to reclaimed humongous objects.
We just need to be more circumspect when processing the SATB buffers
and not assume (and assert) the entries are valid objects until those
tests have been applied, unlike in other cases.

To support this new behavior:

- CMTask::deal_with_reference is now split into an outer part that
performs tests on an object to determine whether it needs to be shaded
grey, and an inner part that performs the shading.

- SATB buffer is changed to perform similar tests on possibly stale
pointers, and once verified to be interesting, then call the shared
inner part to perform the shading.  To enable the new SATB buffer
processing, the closure involved is changed from an ObectClosure to a
new SATBBufferClosure.

Also made ObjPtrQueue::apply_closure_and_empty() public to eliminate
the need for some friends.

CR:
https://bugs.openjdk.java.net/browse/JDK-8075215

Webrev:
http://cr.openjdk.java.net/~kbarrett/8075215/webrev.00/

Testing:
JPRT
Aurora ad hoc GC Nightly and VM quicktests with G1 as the collector
local jtreg with G1 as the collector
local refworkload with G1 as the collector