swscale/ppc: VSX-optimize yuv2422_X

swscale/ppc: VSX-optimize yuv2422_X

./ffmpeg -f lavfi -i yuvtestsrc=duration=1:size=1200x1440 \
          -s 1200x720 -f null -vframes 100 -pix_fmt $i -nostats \
          -cpuflags 0 -v error -

7.2x speedup:

yuyv422
 126354 UNITS in yuv2packedX,   16384 runs,      0 skips
  16383 UNITS in yuv2packedX,   16382 runs,      2 skips
yvyu422
 117669 UNITS in yuv2packedX,   16384 runs,      0 skips
  16271 UNITS in yuv2packedX,   16379 runs,      5 skips
uyvy422
 117310 UNITS in yuv2packedX,   16384 runs,      0 skips
  16226 UNITS in yuv2packedX,   16382 runs,      2 skips