Before adding stack unwinding of interpreted wasm frames, clean up the
respective method a bit.
Replace if-cascade by a switch, and inline the (previously public)
RemoveMaterializedObjectsOnUnwind method.

R=mstarzinger@chromium.org, jarin@chromium.org
BUG=v8:5822

Change-Id: Icf80c4adadc2f43551656ced8e92a67752d5c471
Reviewed-on: https://chromium-review.googlesource.com/453898
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43860}