And then lower it afterwards. This is more future-proof for
multi-return values.

R=titzer@chromium.org
CC=​rossberg@chromium.org

Bug: v8:6672
Change-Id: I6505b049275360c32530992c1db8765254b405c1
Reviewed-on: https://chromium-review.googlesource.com/602036
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47166}