RFR: Heap region sampling overflow in compact matrix

[Aleksey Shipilev]

There is a problem with compact matrix encoding, that overflows the counter array. Crashes the
fastdebug JVM when sampling is enabled.

Fix:
 http://cr.openjdk.java.net/~shade/shenandoah/sampling-matrix-overflow/webrev.01/

Thanks,
-Aleksey