Previously we allowed unreachable returns to be skipped.
This won't work if we strictly follow the spec.

BUG=None
TEST=Manual against more strict wasm validation
R=titzer@chromium.org,rossberg@chromium.org

Review-Url: https://codereview.chromium.org/2668873003
Cr-Commit-Position: refs/heads/master@{#42833}