Use a single CheckMaps node instead of the sequence of LoadField,
ReferenceEqual and CheckIf. This also makes it easier to eliminate
the COW check if there are multiple of them in a row.

R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2216453002
Cr-Commit-Position: refs/heads/master@{#38321}