In a nutshell: The FILTER_KEY builtin is gone, and was replaced by a
simple runtime call to ForInFilter, which does everything and is even
cheaper (because FILTER_KEY used to call into the runtime anyway).
And ForInFilter returns either the name or undefined, which makes it
possible to remove the control flow construction from the AstGraphBuilder,
and thereby make both the initialization and the per-loop code of for-in
optimizable later (in typed lowering).

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1160983004

Cr-Commit-Position: refs/heads/master@{#28711}