Revert "Use Representation::Integer32() for smi types on 32-bit-tagged systems."

Due to performance regression.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/189843006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19709 ce2b1a6d-e550-0410-aec6-3dcde31c8c00