RFR: Remove matrix operations from concurrent mark

[Aleksey Shipilev]

We have discussed this before: matrix is needed for partial collections, partial
collections force update-refs phase, and update-refs rebuilds the matrix.
Therefore, matrix rebuild is not needed during concurrent mark. Moreover, we now
do double work: we first rebuild matrix during concmark, and then do it again
during update-refs! This is excessive.

This removes matrix operations from concurrent mark:
  http://cr.openjdk.java.net/~shade/shenandoah/matrix-remove-cm/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey