Don't force representations in the loop builder.
Additionally, infer None-typed CompareIDAndBranch to Smi+actual input.

R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/16365004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14960 ce2b1a6d-e550-0410-aec6-3dcde31c8c00