Only create spread-related feedback slots when the array literal
actually contains a spread.

Bug: v8:5940
Change-Id: I0afad81d4bf1a86ebc1bf81f1213f680eb22bc49
Reviewed-on: https://chromium-review.googlesource.com/947955
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51721}