[sh/8u] backport 8237632: Shenandoah: accept NULL fwdptr to cooperate with JVMTI and JFR

[Zhengyu Gu]

Please review this backport. The patch is largely inline with original 
patch, but 8u does not have mark word refactoring, HeapWord casting and etc.

One noticeable difference, is that, 8u does not process roots 
concurrently, so there is no chance for heap iteration to encounter dead 
oops.


Bug: https://bugs.openjdk.java.net/browse/JDK-8237632
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8237632-8u/webev.00/

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu