RFR: Allow allocations in pinned regions

[Aleksey Shipilev]

This is partially a regression from FSM change, partially the overlook from the previous code.
Pinned regions should not go to CSet, because they cannot move, but we can definitely allocate in
them. Without this, trying to allocate in Pinned region (after it was added to freeset as Regular)
would fail with "Illegal transition from Pinned to Regular".

Fix:
  http://cr.openjdk.java.net/~shade/shenandoah/region-pinned-alloc/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey