[patch, spu] Implementation of vector unpack for Cell SPU

[Trevor_Smigiel@playstation.sony.com]

Ira,

> I changed the patch according to your comments. I removed vec_extendhisi2
> and used spu_xshw instead. I also used vec_select in vec_unpacks, since
> otherwise I got "warning: this decimal constant is unsigned only in ISO
> C90".

Cool, the vec_select looks much nicer.  In the *_v16qi patterns you put
vec_select:V8HI, which should be vec_select:V8QI.

> 
> Tested on Cell SPU. O.K. for 4.4?

Looks good.  After the minor change above, go ahead.

Trevor