This change allows turbofan to store the index variable in
a Word32 integer; my last change caused the index to be
promoted to float64.

Change-Id: Ia60d16461991ae941229f45d393e924d0dca5bbe
Reviewed-on: https://chromium-review.googlesource.com/878160Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50780}