Adds string type feedback to Ignition's AddWithFeedback code stub, for now only
adding a special case for when both lhs and rhs are strings. This improves
octane's splay by >100%.

BUG=v8:5400

Committed: https://crrev.com/fb4ae2239d37adaf0321165034050316914de708
Review-Url: https://codereview.chromium.org/2392533002
Cr-Original-Commit-Position: refs/heads/master@{#39987}
Cr-Commit-Position: refs/heads/master@{#39996}