For a polymorphic access to o.x we would only merge it into a single
PropertyAccessInfo so far, if x is at the same offset in all maps and
the property index of x (in the descriptor arrays) is the same. But that
doesn't matter for code generation and blocks optimizations even.

BUG=v8:6278,v8:6344,v8:6396
R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2883883002
Cr-Commit-Position: refs/heads/master@{#45294}