[turbofan] Machine operators are globally shared singletons.

TEST=compiler-unittests,cctest
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/547233003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23864 ce2b1a6d-e550-0410-aec6-3dcde31c8c00