RFR(XXS) 8217522: Missing barriers in some java_lang_String assertion code after JDK-8213229

[Zhengyu Gu]

Although the changes are in shared code, but they only affect Shenandoah.

Once JDK-8213229 pushed, we need this patch to unbreak Shenandoah tests. 
The root cause, after JDK-8213229, is that references in stringtable are 
updated concurrently, instead of at safepoints.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217522
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8217522/webrev.00/

Test:
   hotspot_gc, tier1, 2, 3_gc_shenandoah on Linux x64 (fastdebug)

Thanks,

-Zhengyu