Improve smi support in crankshaft
- Recover smi in phis if inputs are smi-typed
- Don't record smi-typed values as pointers

BUG=
R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/16240003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14926 ce2b1a6d-e550-0410-aec6-3dcde31c8c00