If you try to store an object in new space to the Code header, it will
be added to the store buffer, and a DCHECK will fail later, since Code
objects should never occur in the store buffer.

This CL adds DCHECKs to catch such assignments early. Once we handle
this case better, they can be removed again.

R=mstarzinger@chromium.org, ulan@chromium.org
BUG=chromium:674535

Review-Url: https://codereview.chromium.org/2587073002
Cr-Commit-Position: refs/heads/master@{#42142}