When we hit a call to String.prototype.concat builtin, where we can
infer that the receiver is a String and there's exactly one parameter,
which is of type PlainPrimitive, then we can reduce that to a call to
the StringAddStub instead, optionally converting the non-String - but
PlainPrimitive - parameter to a String.

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

Review-Url: https://codereview.chromium.org/2758383002
Cr-Commit-Position: refs/heads/master@{#43957}