RFR: ShenandoahRootProcessor corrupts none Shenandoah VM_Operations

[Zhengyu Gu]

There are actually two bugs:

1. We did not clear bitmap before marking
2. We blindly cast VM_Operation to VM_ShenandoahOperation and set field, 
corrupts none Shenandoah VM_Operations, such as HeapDump and ThreadDump.


Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/vm_op_corruption/webrev.00/

Test:

   tier3_gc_shenandoah


Thanks,

-Zhengyu