RFR (S) 8242273: Shenandoah: accept either SATB or IU barriers, but not both

[Roman Kennke]

Yes, looks good!

Thanks,
Roman


> Bug:
>   https://bugs.openjdk.java.net/browse/JDK-8242273
> 
> The follow-up for JDK-8242054, we need to make sure users cannot enable both SATB and IU barriers
> together. Currently Shenandoah blindly accepts +ShenandoahSATBBarrier
> +ShenandoahStoreValEnqueueBarrier, which might lead to surprising behaviors.
> 
> Since "mode" is what defines the barriers, I moved all relevant barrier checking code there.
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8242273/webrev.01/
> 
> Testing: hotspot_gc_shenandoah {fastdebug,release}
>