Not all interface implementations can handle unreachable code
correctly. The baseline compiler will fail if it tries to pop a value
from an empty stack. Instead of fixing this in the interface (the
baseline compiler in this case), this CL fixes this by not calling the
interface for unreachable instructions.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I567783ce4191ff907b2f90e73b66274728064092
Reviewed-on: https://chromium-review.googlesource.com/715638
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48544}