Don't unconditionally assume that Tagged->Word32 changes are always
truncating independent of the type of the input. Work-around the
remaining issues with for-in by properly renaming the index if it's
not already of appropriate type (happens with generators or OSR).

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2545393002
Cr-Commit-Position: refs/heads/master@{#41484}