RFR 8221435: Shenandoah should not mark through weak roots

[Zhengyu Gu]

Please review this patch that fixes incorrectly marking through weak 
roots in Shenandoah.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221435
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8221435/webrev.00/index.html

Test:
   hotspot_gc_shenandoah on Linux x64 (fastdebug and release)


Thanks,

-Zhengyu