RFR 8231250: Shenandoah: Traversal GC should keep alive weak load from heap

[Roman Kennke]

OK.

We need to have a close look at the interaction between traversal and 
native barriers/conc roots soon.

Roman


> Resubmit:
> Was:  8231250: Shenandoah: Shenandoah assert_correct failed; Object 
> klass pointer
> 
> SAP reported that this patch did not fix the assert failure in original 
> bug report. But the fix is legit, as JDK-8227635 did change the behavior 
> of traversal GC, resulting it may drop keepalive for heap loads.
> 
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8231250
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8231250/webrev.00/
> 
> Test:
>    hotspot_gc_shenandoah (fastdebug and release) on Linux x86_64
> 
> Thanks,
> 
> -Zhengyu
>