The keyed load/store lowering is too aggressive when it comes to element
vs. property access. If we cannot find a cached name on the IC we
automatically assume that it's an element access, i.e. we assume that
the key that is passed to the keyed access must be a valid array index
then. But this is not true for megamorphic keyed load/store ICs, which
do not have a cached name (because the IC saw different names), and thus
use a different mechanism to indicate that it's a non-element access.

Review-Url: https://codereview.chromium.org/2195583002
Cr-Commit-Position: refs/heads/master@{#38155}