RFR (S): Avoid scanning primitive arrays

[Aleksey Shipilev]

Hi,

In many workloads, there are significant number of primitive arrays. We can save
the concurrent mark time by avoiding traversing their oop contents. Which there
are none, but the downcall to oop_oop_iterate_nv still happens.

Patch:
  http://cr.openjdk.java.net/~shade/shenandoah/mark-primitive-arrays/webrev.01/


Improves mark times:

baseline:
 Concurrent Marking =    48.32 s (a =   549133 us) (n =    88)
   (lvls, us =   287109,   505859,   546875,   593750,   755718)

patched:
 Concurrent Marking =    46.95 s (a =   533516 us) (n =    88)
   (lvls, us =   283203,   494141,   523438,   572266,   744296)

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey