For "top digit" (of the result) comparison to be applicable, we must
also check that there are no further digits in the source.

Bug: v8:7150
Change-Id: I6ad317f6f600e11fef59b9907da1055e5586a3a8
Reviewed-on: https://chromium-review.googlesource.com/804639Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49846}