[turbofan] Reenable value numbering.

Value numbering is now limited to eliminatable operators (i.e. operators
that don't throw and don't write), and uses linear probing instead of
separate chaining.

TEST=unittests
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/630423002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24452 ce2b1a6d-e550-0410-aec6-3dcde31c8c00