RFR: 8142402: G1 should not redirty cards in free regions

[Erik Helin]

Hi all,

this patch ensures that G1 does not redirty cards for regions that are
free. This not a problem for product builds, but it causes the card
verification code to fail in debug builds, which is annoying.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8142402

Webrev:
http://cr.openjdk.java.net/~ehelin/8142402/webrev.00/

Testing:
JPRT

Thanks,
Erik