MIPS: Fix debug mode test failures.

This commit fixes a couple of debug mode test failures,
which were introduced by commit 14600.

TEST=mjsunit/mirror-number,mjsunit/mirror-object

BUG=

Review URL: https://codereview.chromium.org/15179005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14699 ce2b1a6d-e550-0410-aec6-3dcde31c8c00