This CL now uses StringCharCodeAt + StringFromCharCode to replace
StringCharAt. Optimizations are easier to implement if we have both
operators; however, if this tanks performance a lot we have to revert.

R=bmeurer@chromium.org

Bug: v8:7531
Change-Id: I75590cc8b8db57715bc2de9f5b98d0878d62a394
Reviewed-on: https://chromium-review.googlesource.com/956134
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51877}