RFR: Prevent accidental UseShenandoahMatrix enabling

[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