This helps if the polymorhpic case has other maps with fast accesses.

In box2d (with high number of iterations and warm-up), we spend about 2.7% of running time in StoreIC; after this change it is only 0.2%.

Review-Url: https://codereview.chromium.org/2494673002
Cr-Commit-Position: refs/heads/master@{#40957}