RFR (S) 8242353: Shenandoah: micro-optimize region liveness handling

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8242353

Webrev:
  https://cr.openjdk.java.net/~shade/8242353/webrev.01/

There are two micro-optimizations in liveness handling:
 - No need to acquire the region eagerly in SH::flush_liveness_cache
 - No need to bypass liveness cache for bulk object separately, we can just use the usual overflow path

This makes a small but visible dent in marking times.

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey