RFR: 8220606: Move ScavengeNMethods unlinking to unregister_nmethod

[Stefan Karlsson]

Hi all,

Please review this patch to move the removal of non-alive nmethods from 
the scavengable nmethods list from the flush/purge phase into the 
unregister_nmethod call.

This will prevent the GCs from scavenging dead nmethods.

http://cr.openjdk.java.net/~stefank/8220606/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8220606

Tested with tier1-3.

Thanks,
StefanK