[turbofan] Add support for keyed access to strings.

This introduces initial support to handle keyed load access to
String primitives. This is accomplished via the existing operators
StringCharCodeAt and StringFromCharCode, which we already use to
optimize String.prototype.charCodeAt and String.fromCharCode.

R=yangguo@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2232483002
Cr-Commit-Position: refs/heads/master@{#38512}