RFR: Fix assertion failure due to race while pre-evacuating Jvmti roots

[Zhengyu Gu]

After pushing pre-evacuation of Jvmti roots, further tests revealed that 
early/other pre-evacuation tasks may race to evacuate oops in Jvmti 
roots, that resulted assertion failure (shenandoah_assert_not_forwarded) 
in ShenandoahIsAliveClosure, use ShenandoahForwardedIsAliveClosure instead.

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


Test:
   tier3_gc_shenandoah and vmTestbase/nsk/jdi
   (fastdebug and release)

Thanks,

-Zhengyu