MIPS: Store doubles before calling into the elements transition stub

Port r16172 (cc2b6204)

BUG=

Review URL: https://codereview.chromium.org/23110002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16182 ce2b1a6d-e550-0410-aec6-3dcde31c8c00