MIPS: Fix loading Float64 constants in turbofan.

TEST=ctest/test-run-machops/RunFloat64SubImm1,
     RunFloat64SubImm2, RunFloat64MulImm, RunFloat64Compare

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/646923003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24536 ce2b1a6d-e550-0410-aec6-3dcde31c8c00