[14] RFR 8237396: JvmtiTagMap::weak_oops_do() should not trigger barriers

[Zhengyu Gu]

Please review this small patch that avoids barriers in 
JvmtiTagMap::weak_oops_do() method.

The method is used by GC and GC expects to see raw oops.


Bug: https://bugs.openjdk.java.net/browse/JDK-8237396
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8237396/webrev.00/

Test:
   hotspot_gc
   vmTestbase_nsk_jvmti
   (fastdebug and release) on x86_64 Linux

   Submit test in progress.

Thanks,

-Zhengyu