RFR: ShenandoahOOMDuringEvacALot

[Roman Kennke]

When I made my prototype I added some diagnostics code to provoke the 
failure path for OOM-during-evac very frequently. This proved to be very 
useful. I extracted this part from that change and post it here because 
I think it's useful, whichever way we go with the bit-mangling patch.

http://cr.openjdk.java.net/~rkennke/oomalot/webrev.00/ 
<http://cr.openjdk.java.net/%7Erkennke/oomalot/webrev.00/>

Ok?

Roman