RFR: 6899049: G1: Clean up code in ptrQueue.[ch]pp and ptrQueue.inline.hpp

[Kim Barrett]

Updates for comments from Per and Thomas:

CR:
https://bugs.openjdk.java.net/browse/JDK-6899049

Webrev:
http://cr.openjdk.java.net/~kbarrett/6899049/webrev.01/
http://cr.openjdk.java.net/~kbarrett/6899049/webrev.01.inc/

Testing of .01 changes:
JPRT