Previously String element access and String.prototype.charAt were
lowered to a subgraph StringFromCharCode(StringCharCodeAt(s, k)),
however that can be fairly expensive both runtime and compile time
wise. The dedicated StringCharAt operator is implemented via a call
to a builtin that does exactly this.

R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2599683002
Cr-Commit-Position: refs/heads/master@{#41909}