RFR (XS) 8240069: Shenandoah: turn more flags diagnostic

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8240069

Webrev:
  http://cr.openjdk.java.net/~shade/8240069/webrev.01/

Regular sweep of flags that are experimental, but have been used as diagnostic. Diagnostic flags are
usually for features that are enabled by default, and are not expected to be disabled, unless
someone is chasing the bug.

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey