RFR (S) 8231947: Shenandoah: cleanup ShenandoahHumongousMoves flag treatment

[Roman Kennke]

Yup! Thanks!

Roman


> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8231947
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8231947/webrev.02/
> 
> This was enabled for a while now. Flag is changed to diagnostic, comment updated, the accessors
> renamed to make more sense.
> 
> Testing: hotspot_gc_shenandoah, new test
>