RFR: 8139890: Refactor self forwarding pointer removal task in G1

[Erik Helin]

Hi all,

this small patch refactors how G1ParRemoveSelfForwardPtrsTask visits
the regions that need self-fowarding pointers removed. An new method,
apply_to_regions, is introduced, which makes it a bit easier to
eventually use G1ParRemoveSelfForwardPtrsTask to remove self-fowarding
pointers not only from the collection set.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8139890

Webrev:
http://cr.openjdk.java.net/~ehelin/8139890/webrev.00/

Testing:
JPRT

Thanks,
Erik