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

[Zhengyu Gu]

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