Instead, return directly when branching to the outermost block. When
falling through the end of the function block, generate a standard
return, otherwise do not generate a return at the end of the function
block.

R=titzer@chromium.org

Bug: v8:8423
Change-Id: I3c5ffacfa4ef7a960d41bea62920bd98e63d78df
Reviewed-on: https://chromium-review.googlesource.com/c/1369958
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58151}