RFR (XS): Pre-clean JDK 9 code to match JDK 8u

[Aleksey Shipilev]

Hi,

This is the first thing in synchronizing JDK 9 and JDK 8u Shenandoah code to
simplify merges:
  http://cr.openjdk.java.net/~shade/shenandoah/jdk9-8u/jdk9-preclean/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey