RFR: 8055702 - Remove the generations array

[Jesper Wilhelmsson]

Hi,

This is the first part of the generation array removal. Here I remove the _gens 
array and introduce _young_gen and _old_gen instead. The main change is in 
GenCollectedHeap::do_collection() which used a loop to iterate through the 
generation array. I extracted the contents of that loop into a new method and 
call it with a reference to the generation to be collected.

Webrev: http://cr.openjdk.java.net/~jwilhelm/8055702/webrev/

Bug: https://bugs.openjdk.java.net/browse/JDK-8055702


More cleanups in this area are coming. Among other things I will remove from 
GenCollectedHeap: _n_gen, SomeConstants, get_gen(), next_gen(), prev_gen(), 
n_gens() and the concept of levels that is passed around all over the place. 
These will be replaced with a more explicit use of young and old as appropriate.

All that is actually already done, it just needs to be split into more 
reviewable changes. Stay tuned.

Thanks,
/Jesper