Instead of branching to the end merge of the outermost block, we should
return directly. This often generates shorter and faster code, since
the merge is omitted.

R=titzer@chromium.org

Bug: v8:6600, v8:8423
Change-Id: Id5e92b05d3fbbcdb69e4a8bf48629d6031d85291
Reviewed-on: https://chromium-review.googlesource.com/c/1358411Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58129}