Table inits can contain imported functions, hence their code will be a
wasm-to-wasm wrapper.
Fix a DCHECK and add a regression test.

R=ahaas@chromium.org

Bug: chromium:817380
Change-Id: I836be589e1ae66839ccd470154c8dea488e6bc1f
Reviewed-on: https://chromium-review.googlesource.com/943107
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51685}