Finally address that long-standing TODO where ConsString allocation in
TurboFan would always go for the two byte map instead of choosing the
one byte map if the inputs are one byte strings.

Bug: v8:5269, v8:7109
Change-Id: Ibcfceaf499ceebef0ef928ebc5f204bcacf29bc0
Reviewed-on: https://chromium-review.googlesource.com/799700Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49744}