This fixes a corner case where the "instance prototype" diverges from
the "non-instance prototype" that we store on the initial map of a
constructor function.

R=bmeurer@chromium.org
TEST=mjsunit/regress/regress-crbug-703610
BUG=chromium:703610

Change-Id: I30a19ae621e10b512215ffb191ce00d030941440
Reviewed-on: https://chromium-review.googlesource.com/458396Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44008}