MIPS: Fix r21780 - “Fixed flooring division by a power of 2, once again...”

Initialize scratch register for all possible code paths.

TEST=mjsunit/compiler/division-by-constant.js
BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/333713002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21822 ce2b1a6d-e550-0410-aec6-3dcde31c8c00