The read-modify-write (RMW) optimization looks for patterns like this:

  a = Load addr
  b = <op> a, other
  Store b, addr

and essentially transforms them into this:

  RMW <op>, addr, other

This CL also applies the transformation when the middle instruction is
  b = <op> other, a
and <op> is commutative.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4095
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1193103005