Unify canonicalization of HAdd/HSub/HMul a bit.

HDiv/HMul are a slightly different story and will be handled in a separate CL.

Review URL: https://codereview.chromium.org/14296013

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14322 ce2b1a6d-e550-0410-aec6-3dcde31c8c00