MIPS64: Fix bug in super assignment from r24570.

TEST=mjsunit/harmony/super.js
BUG=
R=akos.palfi@imgtec.com

Review URL: https://codereview.chromium.org/677813002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24889 ce2b1a6d-e550-0410-aec6-3dcde31c8c00