RFR: Disable matrix for good when -UseShenandoahMatrix

[Aleksey Shipilev]

Hi,

Turns out, clearing matrix is costly, and it sometimes happens during "prepare
evac" at final mark pause:

[56.054s][info][gc,stats]   Prepare Evacuation        =     0.22 s
    (a =    16725 us) (n =    13)
     (lvls, us =      137,    16992,    17383,    18555,    19583)


This fix disables matrix operations for good when matrix is disabled:
  http://cr.openjdk.java.net/~shade/shenandoah/matrix-disable-for-good/webrev.01/

This returns final mark pauses to normal. We would need to look into fixing this
performance pothole for the case the matrix is enabled and needed by GC.

Testing: hotspot_gc_shenandoah, benchmarks

Thanks,
-Aleksey