After the FallThruTo in kExprEnd, the current block {c} is never
unreachable. Hence, the check for {c->unreachable} afterwards can be
removed.
In the loop case, the {TypeCheckFallThru} already adds entries for
non-existing values to the stack, so no need to {PushEndValues}.
Also, add more tests for the loop case.

R=titzer@chromium.org

Change-Id: I8737affaeed2ea663bd6ddafa36532ca9a7379bb
Reviewed-on: https://chromium-review.googlesource.com/645859Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47765}