Array.prototype.map is currently not preserving PACKEDness. Use a
for-loop instead.

Bug: v8:7980
Change-Id: I08aff1cbcd84b9de260a5a1e2c68b9cfb5c3d888
Reviewed-on: https://chromium-review.googlesource.com/c/1280329
Commit-Queue: Hai Dang <dhai@google.com>
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56657}