Also add a new fast-path for String.fromCodePoint.

R=neis@chromium.org

Bug: v8:7570, v8:7340
Change-Id: I6cd6e6fc98943588ecd646f24fcda043d4033ab0
Reviewed-on: https://chromium-review.googlesource.com/978244Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52183}