[Mono-dev] setters and getters for mono.simd

[Jerry Maine]

I was looking for over the simd specs and I found out something very
interesting.

PEXTRW  (Extract Packed Word) and PINSRW (Packed Insert Word)

These two could be used for the getters and setters for Vector4f (though
one would have to use them in pairs) and the good thing is that  they
are plain SSE instructions, not SSE2 nor SSE3.

It would save us from having to off load and reload the entire 128 bits
to and from the register.

What do you guys think?