This makes sure that the lowering of {JSCreate} operator during create
lowering is only applied to operations where both target and new.target
are known to be constructors.

R=jarin@chromium.org
TEST=mjsunit/regress/regress-crbug-768080
BUG=chromium:774780,chromium:768080

Change-Id: I55a582a3453bba7e14655b594b7714a3940eeaae
Reviewed-on: https://chromium-review.googlesource.com/725332Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48680}