In the presence of bigints, this optimization is no longer valid.

Bug: v8:6791
Change-Id: I996ac78f8ae4aef5494dd0089374d04c6db6e72f
Reviewed-on: https://chromium-review.googlesource.com/796070Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49725}