RFR: 8227086: Use AS_NO_KEEPALIVE loads in HeapDumper

[Stefan Karlsson]

Hi all,

Please review this patch to read objects with AS_NO_KEEPALIVE in the 
HeapDumper.

http://cr.openjdk.java.net/~stefank/8227086/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8227086

Found this while running some extra verification code through our 
barriers. This is one place where we use ON_UNKNOWN_OOP_REF without 
AS_NO_KEEPALIVE. The current code isn't wrong, but we could just as well 
use the more light-weight load barrier here.

Tested with tier 1-7

Thanks,
StefanK