RFR (S) 8241743: Shenandoah: refactor and inline ShenandoahHeap::heap()

[Roman Kennke]

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8241743
> 
> ShenandoahHeap::heap() is used on critical performance fastpaths, and should be properly inlined.
> Instead of going via Universe::heap(), we can just pull it off our own static field. (ZGC does the
> same). heap_no_check() is not needed anymore, because we don't do any additional checks that make
> performance worse.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8241743/webrev.01/
> 
> Testing: hotspot_gc_shenandoah; Linux x86_64 {slowdebug, fastdebug, release} builds without PCH

That makes sense. Patch looks good!

Roman