MIPS: Tweak StoreKeyed.

Port r16771 (536eb66)

Original commit message:
Avoid corrupting its input in some cases.

BUG=none
TEST=test/mjsunit/lithium/StoreKeyed*.js
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/23537053

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16779 ce2b1a6d-e550-0410-aec6-3dcde31c8c00