8166229: Eliminate ParNew's use of klass_or_null()

[Kim Barrett]

Please review this change to ParNew (the young-gen collector paired
with CMS), eliminating it's use of klass_or_null(), instead using
list_ptr_from_klass().

CR:
https://bugs.openjdk.java.net/browse/JDK-8166229

Webrev:
http://cr.openjdk.java.net/~kbarrett/8166229/webrev.00/

Testing:
Local jtreg tests with the following additional options:
 -XX:+UseConcMarkSweepGC
 -XX:-UseCompressedOops
 -XX:-ParGCUseLocalOverflow
 -XX:+ParGCWorkQueueOverflowALot
and temporary logging on the code path where klass_or_null was being
used, to verify testing reached the changed code.