This is a cleanup in preparation to implement calling imported
functions via the wasm interpreter.
For imported functions, we do not create entries in the
interpreter_code_ vector any more.

I also simplified the interface and removed unused or redundant return
values. More things are now DCHECKed instead of bailing out.

Also, we previously had two PushFrame methods: One is supposed to
initialize the interpreter from external code (i.e. adds the first
frame to the stack), the other one is used to push new frames on the
frame stack for called functions. This CL renames the first to
InitFrame, and makes it use the second one. The other remaining user is
the DoCall method.

R=titzer@chromium.org
BUG=v8:5822

Change-Id: Id09ff1e3256428fbd8c955e4664507a0c3167e53
Reviewed-on: https://chromium-review.googlesource.com/453482
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43793}