RFR: 8212921: Relax code cache oop verification in ZGC

[Erik Österlund]

Hi,

With concurrent class unloading, the normal state of affairs for ZGC is 
that nmethods have messed up oops in all GC safepoints, and fix them up 
during concurrent phases. Therefore, Universe::verify needs to relax the 
assumption that oops are okay for ZGC.

Note that during Universe::verify, the oops in the nmethods were 
redundantly verified. Once by walking the oops in the code cache, and 
once when calling nmethod::verify(), which did the same verification 
again. I removed the latter.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8212921/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8212921

Thanks,
/Erik