Executing the |end| opcode of a loop assumed that the stack height was
being reset to the height at start of the loop. Hence we were ignoring
the arity of the loop.
During computation of the side table, the arity of the label associated
with the loop was explicitly set to 0, such that a |br| instruction to
that label would not transfer any values.
It turns out though that we need to remember the arity in order to
precompute the correct stack height when executing the |end| opcode of
a loop.
Also, add a regression test.

R=rossberg@chromium.org
BUG=chromium:716936

Change-Id: Ib3a559998f1ce5f8fcd7b94af1426637b3e48f86
Reviewed-on: https://chromium-review.googlesource.com/493286
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45041}