RFR(S) 8222425: Shenandoah: Move commonly used closures to separate files

[Zhengyu Gu]

Please review this patch that refactors some shared closures to separate 
files.

Bug: https://bugs.openjdk.java.net/browse/JDK-8222425
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8222425/webrev.00/

Test:

hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu