We currently assume that all prototype maps are stable, which is
not guaranteed for certain keyed access patterns. So we explicitly
disallow optimizing the element access there for now.
BUG=chromium:557807, v8:4470
R=jarin@chromium.org
LOG=n
Review URL: https://codereview.chromium.org/1456973004
Cr-Commit-Position: refs/heads/master@{#32101}
