RFR [8u]: Revert G1 changes and bring shared BitMap

[Aleksey Shipilev]

In 8u, in contrast to 9 and 10, Shenandoah actually calls into g1 code for
bitmaps. So, reverting G1 changes there needs to coincide with bringing shared
BitMap from 9.

This is 8u-specific:
  http://cr.openjdk.java.net/~shade/shenandoah/revert-g1-jdk8/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey