This makes sure we are using the right signedness - either the input
specifies signedness or the use must declare it does not care.

This requires some hole-punching in for-in so that the
representation selector understands the smi-ness of index.
Moreover, phi needs to pass the truncation along (maybe
slightly scary).

Review URL: https://codereview.chromium.org/1455123002

Cr-Commit-Position: refs/heads/master@{#32081}