The optimized code for %ArrayIteratorPrototype%.next for holey arrays
was wrong, since it would first store the [[NextIndex]] and then check
whether it hit a hole. However in that case TurboFan doesn't have any
point to deoptimize to, so we need to perform the side-effecting stores
only after all checks are done.

Bug: v8:7510, v8:7514, chromium:819086
Change-Id: I0214c7124833286113e4dc7403ddc20a82fa8da3
Reviewed-on: https://chromium-review.googlesource.com/950723Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51753}