RFR: 8220609: Cleanups in ScavengableNMethods

[Stefan Karlsson]

Hi all,

Please review this patch to clean up the ScavengableNMethods code.

http://cr.openjdk.java.net/~stefank/8220609/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8220609

- Some renaming of functions and parameters

- Unification of prune_nmethods and scavenge_nmethods_do

- Removal of development logging of scavengable oops.

- Removed is_alive checks, since all operations nmethods in this list is 
alive.

- Removed fix_relocation check, since this code is only supposed to use 
from scavenging GCs, that do relocations. We removed the other use-case 
with "JDK-8220342: Remove scavenge_root_nmethods_do from 
VM_HeapWalkOperation::collect_simple_roots".

Testing: tier1-3

Thanks,
StefanK