RFR: 8166276: Refactor gen_process_roots to allow simpler fix for 8165949

[Stefan Johansson]

Hi all,

While working on JDK-8165949 [0] I realized that gen_process_roots 
needed to be cleaned/split up to allow it to be fixed. This work had in 
big parts already been done as part of an enhancement [1] that never got 
pushed. That enhancement included another cleanup as well but I have 
split it out to only include the necessary part in this fix. It splits 
gen_process_roots into two different versions, one used for young 
collections and one for old. This make the two methods much easier to 
follow and removes some special cases. I will post a second review for 
the actual bug-fix.

Webrev:
http://cr.openjdk.java.net/~sjohanss/8166276/hotspot.00/

Subtask:
https://bugs.openjdk.java.net/browse/JDK-8166276

Testing:
* JTREG tests locally
* RBT on HS-gc tier 2-4

Thanks,
Stefan

[0] https://bugs.openjdk.java.net/browse/JDK-8165949
[1] https://bugs.openjdk.java.net/browse/JDK-8072809