replace a few mov + psrlq with pshufw, there are more cases which could...

replace a few mov + psrlq with pshufw, there are more cases which could benefit from this but they would require us to duplicate some functions ...
the trick is from various places (my own code in libpostproc, a patch on the x264 list, ...)

Originally committed as revision 4608 to svn://svn.ffmpeg.org/ffmpeg/trunk