RFR: 8202639: Use concrete Generation classes in SerialHeap and CMSHeap

[Stefan Karlsson]

Hi all,

Please review this patch to add and use young_gen() and old_gen() 
functions that add returns the concrete Generation classes.

http://cr.openjdk.java.net/~stefank/8202639/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8202639

GenCollectedHeap contains fields for the _young_gen and _old_gen, with 
associated young_gen() and old_gen() accessors. These accessors return 
Generation*s.

These Generations are always DefNewGeneration and TenuredGeneration for 
the Serial GC, and ParNewGeneration and ConcurrentMarkSweepGeneration 
for CMS.

I propose that we expose two new accessors that return the concrete 
Generation classes in SerialHeap and CMSHeap.

Working with these concrete classes will allow us to easier split the 
Serial and CMS code. Specifically, this makes it possible to remove 
ALL_SINCE_SAVE_MARKS_CLOSURES and associated macros. See:
https://bugs.openjdk.java.net/browse/JDK-8202641

Thanks,
StefanK