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

[Roman Kennke]

Ok. Nice!

Patch looks good!
Roman

> 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
>