MIPS: Fix unaligned address of double.

TEST=mjsunit/debug-evaluate-locals-optimized-double

BUG=

Review URL: https://codereview.chromium.org/14631016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14828 ce2b1a6d-e550-0410-aec6-3dcde31c8c00