RFR: JDK-8064473 - Improved handling of age during object copy in G1

[Staffan Friberg]

Hi,

Small update to object copying in G1 to reduce the number of times age 
has to be read and also make sure the markOop can be trusted by reading 
and saving it prior doing the forwarded check. This change is broken out 
from the PLAB event RFE.

bug: https://bugs.openjdk.java.net/browse/JDK-8064473
webrev: http://cr.openjdk.java.net/~sfriberg/JDK-8064473/webrev.01

Thanks,
Staffan