RFR: Mark-compact adjust pointers can be specialized with UseShenandoahMatrix

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/mark-compact-matrix-cond/webrev.01/

This saves some time doing Full GC in our default mode.

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey