Split pextrw macro-spaghetti into several opt-specific macros, this will make

Split pextrw macro-spaghetti into several opt-specific macros, this will make
future new optimizations (imagine a sse5) much easier. Also fix a bug where
we used the direction (%2) rather than optimization (%1) to enable this, which
means it wasn't ever actually used...

Originally committed as revision 24507 to svn://svn.ffmpeg.org/ffmpeg/trunk