Only rewind destructuring assignments if we actually preparsed
the arrow function. For the case of single-expression bodies,
we don't preparse, but we were previously erroneously rewinding.

Bug: v8:6970
Change-Id: I38e15a8a5bdb05abee3bafe7bbd7736b55a6950b
Reviewed-on: https://chromium-review.googlesource.com/733950Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48883}