TRY: pass all SIMD vecs by value, not reference.

Actually, since they're inlined anyway, this results in the same machine
code. And even if it wasn't inlined, passing an 8-byte address
(reference) doesn't offer much advantage over passing a 16 byte SIMD
value, and passing by value makes the compiler certain that no aliasing
could occur. It probably makes no difference and is just a matter of
aesthetics.