When StringAdd builtin is used to concatenate two primitive values,
we know that the operation might throw, but cannot trigger any other
observable side effect, so it shouldn't flush the LoadElimination state.

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

Review-Url: https://codereview.chromium.org/2645523002
Cr-Commit-Position: refs/heads/master@{#42489}