RFR(sh/jdk11): Shenandoah: Isolate shared-code changes

[Roman Kennke]

Here comes a batch of isolating remaining simple Shenandoah-induced
shared-code changes. The idea is that when building without Shenandoah
GC should compile exactly as it would with current jdk11u.

Webrev:
http://cr.openjdk.java.net/~rkennke/shjdk11-isolate-shared/webrev.00/

Testing: hotspot_gc_shenandoah, with/without Shenandoah, 32/64 bit.

Ok?