MIPS: Move failing ASSERT to a more sane place.

Port r19095 (caf48ad)

Original commit message:
Objects can actually be stored into themselves. This fails when no write
barrier is needed (eg, the object was just allocated).

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/149343003

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19122 ce2b1a6d-e550-0410-aec6-3dcde31c8c00