This was discovered on the babel test of the web-tooling-benchmark,
which suffers from a high %KeyedGetProperty overhead, and most of
these calls come from the fact that the KeyedLoadIC_Megamorphic bails
out to the runtime call for all String instance types. Just handling
in-bound accesses to characters reduces the overhead incurred by
%KeyedGetProperty from roughly 9% to roughly 2% only.

This reduces the number of runs per second on the babel test by around
7-8% on average.

Bug: v8:6936, v8:7014
Change-Id: I0dc247d7d6457c7032636d2852cb54cef1b24979
Reviewed-on: https://chromium-review.googlesource.com/743012Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49021}