In the case of an indirect call to an imported function, the target
instance stored in the IFT was actually wrong.

Bug: chromium:834619
Change-Id: Id2ac4158335ecf2b58e1983ce37df852a9ebd1b2
Reviewed-on: https://chromium-review.googlesource.com/1030174
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52831}