The temp_instance is not used but had an enormous performance impact.
The instantiation time of the zengarden WebAssembly module improved now
from 12 seconds to less than a second.

R=clemensh@chromium.org
BUG=v8:6146

Change-Id: Ibac7809b3b6092032abd77ed94f639c09f62d816
Reviewed-on: https://chromium-review.googlesource.com/458918
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44114}