RFR (S) 8241841: Shenandoah: ditch one of allocation type counters in ShenandoahHeapRegion

[Roman Kennke]

Looks good.

Thanks,
Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8241841
> 
> We currently count the allocation by type: TLAB, GCLAB, shared allocs. All together, they should add
> up to the "used" space in the region. That means we can ditch one of the counters, and infer it from
> the already tracked "used" size.
> 
> "Shared" counter seems to be the most profitable to go: it usually means either a small allocation
> that does not need another small roadbump on allocation path, or the humongous allocation that does
> increments for every region in the humongous chain.
> 
> This saves 4..8 bytes per region, and drops x86_32 size to 64 bytes (1 cache-line) without padding.
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8241841/webrev.01/
> 
> Testing: hotspot_gc_shenandoah
>