Fix DoubleStackSlot-to-DoubleStackSlot moves on ia32. Unify platform-independent code.

BUG=173907

Review URL: https://codereview.chromium.org/12207063

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13617 ce2b1a6d-e550-0410-aec6-3dcde31c8c00