RFR (XS): Prune is_in call from in_cset_fast_test

[Aleksey Shipilev]

Hi,

The profiles on SPECjvm2008 show that ShenandoahHeap::is_in is in
hotspots. Further analysis shows the most dominating use is
in_cset_fast_test. It seems to me the ::is_in use there is redundant and
should be instead turned into the assert -- compiled code that dubs this
test blindly trusts it anyway.

Webrev:
  http://cr.openjdk.java.net/~shade/shenandoah/in-cset-opt/webrev.01/

Testing: fastdebug/hotspot_gc_shenandoah

Thanks,
-Aleksey