RFR 8231250: Shenandoah: Shenandoah assert_correct failed; Object klass pointer

[Zhengyu Gu]

Many thanks to Christroph Langer (SAP) for reporting and helping to 
verify the problem.

The initial report was recorded in JDK-8230483, unfortunately, it did 
not fix the problem.

The root cause is that JDK-8227635 work changed behavior of traversal 
GC, which is required to always enqueue new value.

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
   SAP internal tests


Thanks,

-Zhengyu