RFR: Prevent accidental UseShenandoahMatrix enabling

[Roman Kennke]

Am 09.02.2018 um 17:39 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/matrix-diagnostic/webrev.01/
>
> Lennart had accidentally enabled UseShenandoahMatrix with traversal GC, and it is known to break at
> least in two places in ShenandoahBarrierSet (see the patch for new asserts). Demoting
> UseShenandoahMatrix to diagnostic option should help to prevent this in future.
>
> Testing: hotspot_gc_shenandoah (in progress, maybe tests are missing UnlockDiagnostic... somewhere)
>
> Thanks,
> -Aleksey
>
>

Looks good to me.