This patch removes unnecessary scope creation for C-style, for-in, and
for-of loops containing var declarations. Only loops with LET or CONST
declarations require additional scoping up-front.

After this patch lands, I intend to apply this simplification (as well
as that from fa15ba5a) to for-await loops.

Bug: v8:6724
Change-Id: I9962432d1e059d8eefb577e7b512bc2321a03140
Reviewed-on: https://chromium-review.googlesource.com/619987Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47436}