Optimize String.p.split for the case when the separator is empty and
the subject is a direct one-byte string.

Bug: v8:7103
Change-Id: Ica277d2c426679a1f77a1ef8ecb523bd596f65fb
Reviewed-on: https://chromium-review.googlesource.com/1045950
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53260}