And add some tests for (seemingly) previously uncovered cases.
The additional check for unreachable code is not needed any more, since
polymorphic stack values get assigned a specific type on their first
use or validation anyway. Hence the first entry in the br_table will
assign specific types to all polymorphic stack values, and type checking
will fail if later entries do not match.

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

Change-Id: I1d0f91f927a2aa5186f874112e91ebffa1f1b3a7
Reviewed-on: https://chromium-review.googlesource.com/675405Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48109}