Extract GlobalValueNumberer and helper classes from hydrogen.cc and move to hydrogen-gvn.cc.

BUG=

Review URL: https://codereview.chromium.org/16095004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14824 ce2b1a6d-e550-0410-aec6-3dcde31c8c00