Move ConstantPropagator from flow_graph_optimizer.cc/.h into separate files.

flow_graph_optimizer.cc is becoming unnecessarily large, so
I'm  moving self-contained optimization passes into
separate files.

In a separate CL I'll also move load/store optimization into
separate files.

R=srdjan@google.com

Review URL: https://codereview.chromium.org//820883004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42567 260f80e4-7a28-3924-810f-c04153c831b5