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

[Aleksey Shipilev]

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

-- 
Thanks,
-Aleksey