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

The included regression test flushes out another bug in "TruncateToNBits",
so that gets fixed here too (in contrast to the first landing attempt).

This reverts commit cb9e7af4.

Bug: v8:7150
Change-Id: Id631b1ae79e60b8e85ed4667e246a64c46765f2b
Reviewed-on: https://chromium-review.googlesource.com/807348Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49877}