For additions like a+'' or ''+a where we have String feedback on the
JSAdd, we can drop the concatenation and just check that a is a valid
String already (via CheckString).

BUG=v8:6259
R=petermarshall@chromium.org

Review-Url: https://codereview.chromium.org/2894563002
Cr-Commit-Position: refs/heads/master@{#45395}