RFR: 8075466: Address not aligned in Klass::decode_klass_not_null

[Kim Barrett]

> The problem is that a SATB queue may contain a stale reference to an
> eagerly reclaimed humongous object.  Assert-conditional verification
> during full queue processing that all entries are oop's can now fail.
> 
> We deal with this by eliminating the pre-filtering verification
> (removed call to ObjPtrQueue::verify_oops_in_buffer and removed that
> now unused function).  Instead, ObjPtrQueue::filter has been revised
> to be more careful about oop-ness assumptions, and to assert the
> oop-ness of entries that are retained.

Here's an update, modified per Bengt's and Eric's comments.

Eliminated unnecessary cast to HeapWord**.
Removed bogus comment.
Fixed formatting of "if" statement.

CR:
https://bugs.openjdk.java.net/browse/JDK-8075466

Webrev:
http://cr.openjdk.java.net/~kbarrett/8075466/webrev.01/

Incremental Webrev:
http://cr.openjdk.java.net/~kbarrett/8075466/webrev.01.incr/

Testing:
JPRT