Revert "[turbofan] Merge GenericNode with Node."

This reverts commit a238443c00ebbd8afb48ee7ad336e0ff163c84b5 (r24915)
for tanking benchmarks.

TBR=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/684693002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24916 ce2b1a6d-e550-0410-aec6-3dcde31c8c00