Ideally we would have a dedicated MachineRepresentation for Smis during
representation selection and use that to properly optimize ObjectIsSmi
(and other ObjectIs<Type> predicates), but that will take some time to
get that done. So in the meantime we can just do simple (local) strength
reduction on ObjectIsSmi to avoid Smi checks in the simplest cases at
least.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2047213002
Cr-Commit-Position: refs/heads/master@{#36809}