This propagates the existing type of a {JSAdd} node back to the newly
created {Allocate} node. There are cases where said type is {None}.

R=jarin@chromium.org
TEST=mjsunit/regress/regress-crbug-772720
BUG=chromium:772720

Change-Id: Iab18d2108a789b51db4e405f7f335c5c0ca6f686
Reviewed-on: https://chromium-review.googlesource.com/708796Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48493}