MIPS: Fix computed properties on object literals with a double as propertyname.

Port r22255 (cf84805)

BUG=390732
LOG=y
R=palfia@homejinni.com

Review URL: https://codereview.chromium.org/370363002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22256 ce2b1a6d-e550-0410-aec6-3dcde31c8c00