[turbofan] Merge GenericNode with Node.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/676353002

Cr-Commit-Position: refs/heads/master@{#24915}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24915 ce2b1a6d-e550-0410-aec6-3dcde31c8c00