Request for review: 7052429: G1: Avoid unnecessary scanning of humongous regions during concurrent marking

[Tao Mao]

7052429: G1: Avoid unnecessary scanning of humongous regions during 
concurrent marking
https://jbs.oracle.com/bugs/browse/JDK-7052429

webrev:
http://cr.openjdk.java.net/~tamao/7052429/webrev.00/

changeset:
Avoid wasted work of _nextMarkBitMap->iterate() when the current region 
is a humongous region and the first bit is NOT marked, whereas the case 
where a humonguous region with its first bit marked is properly handled 
in iterate() routine.