RFR: Fixed two string dedup issues

[Zhengyu Gu]

Fixed two issues:

1) Missing string dedeup on shenandoah write barrier fast path.

2) Missing dedup table reference update on piggyback reference update path.


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

Test:
   hotspot_gc_shenandoah fastdebug and release on x64 and aarch64.

Thanks,

-Zhengyu