The AllowUndefinedAsNaN optimization is pretty tricky, and more extensive
than
just kElementsCantBeAdded.
kElementsCantBeAdded will trigger when SetFastElement is applied to exactly
Array.prototype. We won't be able to warm up an IC for Array.prototype
using a
different object, since it has a unique map. The same goes for Crankshaft.
(Unlike in your case).
Additionally, if you do something like Array.prototype.length = 1, it'll
deoptimize due to a prototype chain change, since it will make the map go
from
FAST_SMI_ELEMENTS to FAST_HOLEY_SMI_ELEMENTS. The prototype-chain check is
added, but will most likely be elided and replaced by prototype dependency.
https://codereview.chromium.org/198533007/
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