Also support inlining the builtins String.prototype.charCodeAt and
String.prototype.charAt if the index type is not statically known
to be in the Unsigned32 range, but in anything in Integral32 plus
minus zero and NaN.

R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2597913002
Cr-Commit-Position: refs/heads/master@{#41913}