RFR (S) 8241844: Shenandoah: rename ShenandoahHeapRegion::region_number

[Aleksey Shipilev]

RFR:
  https://bugs.openjdk.java.net/browse/JDK-8241844

ShenandoahHeapRegion::region_number is too verbose of the name, plus most of the code already treats
it as "index" in the local variable. Can rename it for consistency. I am not sure whether it should
be "idx" or "index".

Candidate fix:
  https://cr.openjdk.java.net/~shade/8241844/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey