We need to explicitly rule out negative indices for the out-of-bounds
case, otherwise we can end up with a monomorphic KeyedLoadIC that allows
OOB accesses, but doesn't properly check whether there are properties
with negative integer names on the receiver.

Bug: chromium:784835
Change-Id: Ic3ef5438b76094f024de0c6348183fb62b32088c
Reviewed-on: https://chromium-review.googlesource.com/774278Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49396}