RFR: Avoid evacuating filler objects in Shenandoah

[Zhengyu Gu]

Let's get Shenandoah part in before I mess around with other GCs.

Using fake filler array klass seems to have less side-effects, 
especially on upstream collectors.

The drawback is that, we still evacuate small object fillers, since fake 
filler object is not easy.

I ran hotspot_gc tests (fastdebug and release) without any failures, so 
it has no ill-effects on other collectors.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/sh_filler/webrev.00/index.html

Thanks,

-Zhengyu