RFR: JDK-8057632 - Remove auxiliary code used to handle the generations array

[Jesper Wilhelmsson]

Hi,

The generation array removal enabled several other cleanups. I have several 
changes in queue to perform these cleanups one at a time.

In this change the methods next_gen(), prev_gen() and get_gen() are removed and 
calls are replaced with explicit calls to young_gen() and old_gen().

Please have a look.

Bug: https://bugs.openjdk.java.net/browse/JDK-8057632
Webrev: http://cr.openjdk.java.net/~jwilhelm/8057632/webrev.02/

Thanks!
/Jesper