Remove prepXMM() and cgBinaryDblOp()

Summary: prepXMM() was guaranteed to always be passed Dbl args
assigned to virtual registers, turning the whole thing
into a nop. Consequently, cgBinaryDblOp became a nop too,
so just hand-inlined it everywhere. Removed a few
unnecessary copies too.

Reviewed By: @ottoni

Differential Revision: D1548190
(I inspected each one, and it's just the FTP tests which
are failing many diffs)