RFR: Use CollectedHeap::base() instead of ShenandoahHeap::first_region_bottom()

[Roman Kennke]

I found that CollectedHeap has a base() method which seems identical to
ShenandoahHeap::first_region_bottom(). This patch changes all uses of
the latter to the former. Ok?

http://cr.openjdk.java.net/~rkennke/heapbase/webrev.00/

Test: hotspot_gc_shenandoah, jmh-specjvm

Roman