Cleanup %ForInPrepare runtime entry, and unify common logic with
%ForInEnumerate (renamed from %GetPropertyNamesFast). Also introduce
a TupleType to properly type JSForInPrepare and its projections w/o
special hacks in the Typer. And fix %ForInNext and JSForInNext to be
consistent with fullcodegen again (after the proxy refactorings last
quarter).

R=jarin@chromium.org
BUG=v8:3650
LOG=n

Review URL: https://codereview.chromium.org/1631583002

Cr-Commit-Position: refs/heads/master@{#33487}