And related call sites.

Drive-by: add IntPtrZero() and SmiZero() helpers.

Bug: v8:8015
Change-Id: I83195cf2da858f8d35f25c9a1aa5c40c81df78b9
Reviewed-on: https://chromium-review.googlesource.com/1162169
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54918}