RFR: Use leftmost region in GC allocations

[Aleksey Shipilev]

On 02/06/2018 09:06 PM, Dominik Inführ wrote:
> Yes, but AFAIU idx is of type size_t. So if _leftmost is 0, we would have idx >= 0, which is always
> true for unsigned types and we would have an endless loop.

Right! Unsigned data types, argh.

Let's go with this variant:
  http://cr.openjdk.java.net/~shade/shenandoah/freeset-leftmost/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey