RFR (S) 8241740: Shenandoah: remove ShenandoahHeapRegion::_heap

[Roman Kennke]

Looks good, thanks!

Roman


> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8241740
> 
> It is not seem to worth it to drag the _heap field in every region. Almost all the places where it
> is used is not performance-critical, and ShenandoahHeap::heap() is actually as fast, especially
> after JDK-8241743. Ditching this field saves 8 bytes per region.
> 
> I had to move seqnum_last_alloc_mutator to avoid including shenandoahHeap.inline.hpp in a generic
> header.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8241740/webrev.02/
> 
> Testing: hotspot_gc_shenandoah, eyeballing shenandoahHeapRegion.o objdumps
>