RFR: Optimize ShMatrix::clear_(in|out)bound

[Aleksey Shipilev]

We know from sore experience that these methods are slow, and they are called in
our critical paused phases in experimental partial and update-ref code. It makes
sense to optimize it:
  http://cr.openjdk.java.net/~shade/shenandoah/matrix-clear-inoutbound/webrev.01/

Improves update-refs:

[56.765s][info][gc,stats] Pause Post Update References =
   0.11 s (a = 18277 us) (n =     6)
     (lvls, us =    16602,    16602,    16992,    19141,    20516)

[56.784s][info][gc,stats] Pause Post Update References =
   0.04 s (a = 6097 us) (n =     6)
     (lvls, us =     5488,     5488,     5723,     6309,     6958)

Improves partial:

[56.887s][info][gc,stats] Partial GC =
   2.75 s (a =    58532 us) (n =    47)
     (lvls, us =    48242,    49219,    50195,    58594,   154002)

[56.911s][info][gc,stats] Partial GC =
   2.63 s (a =    55895 us) (n =    47)
     (lvls, us =    45117,    46484,    47656,    58742,   149117)


-Aleksey