This matches current Crankshaft/fullcodegen behavior more closely and
thus reduces the chances that we run into unnecessary polymorphism due
to the field representation tracking in our object model.

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

Review URL: https://codereview.chromium.org/2367593003 .

Cr-Commit-Position: refs/heads/master@{#39655}