This fixes cases where falling off the end of a non-void function was
accepted as a valid asm.js module. This led to translation failures in
the WASM code. Only void functions are allowed to fall off the end.

R=clemensh@chromium.org
TEST=mjsunit/asm/return-types
BUG=chromium:719286

Change-Id: I7b1c9ba5381b87a23cf0a2171bee5e5f5e8cd9de
Reviewed-on: https://chromium-review.googlesource.com/500312
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45221}