RFR(S) 8225801: Shenandoah: Adjust SA to reflect recent forwarding pointer changes

[Zhengyu Gu]

Please review this patch, that adjusts SA to reflect removal of extra 
header for forwarding pointer.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225801
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225801/webrev.00/
Test:
  hotspot_gc_shenandoah
  Manual test.


Thanks,

-Zhengyu