Extract String feedback on Add operation and utilize to lower ConsString
creation in JSTypedLowering when we know that a String addition will
definitely result in the creation of a ConsString.

Note that Crankshaft has to guard the potential length overflow of the
resulting string with an eager deoptimization exit, while we can safely
throw an exception in that case.

Also note that the bytecode pipeline does not currently provide the
String feedback for the addition, which has to be added.

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

Review-Url: https://codereview.chromium.org/2354853002
Cr-Commit-Position: refs/heads/master@{#39540}