Don't compute the ToNumber conversion of long strings as it is
expensive.

Bug: v8:7790
Change-Id: Ief104f9229f7f633e31d26c98ed24b08a161525b
Reviewed-on: https://chromium-review.googlesource.com/1177719Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55168}