MIPS: Fix [de]serialize problem of root objects.

BUG
TEST=mjsunit/deserialize-reference
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/492303004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23487 ce2b1a6d-e550-0410-aec6-3dcde31c8c00