MIPS: Tweak LoadKeyed.

Port r17042 (9ade336)

Original commit message:
Avoid corrupting its input.

BUG=none
TEST=none
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/25563002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17058 ce2b1a6d-e550-0410-aec6-3dcde31c8c00