Review request (XS) 7160613: VerifyRememberedSets doesn't work with CompressedOops

[Stefan Karlsson]

http://cr.openjdk.java.net/~stefank/7160613/webrev/

CheckForUnmarkedOops uses oopDesc::load_decode_heap_oop_not_null, which 
asserts because some oops are in fact pointing to NULL. I changed it to 
use oopDesc::load_decode_heap_oop instead.

StefanK