FFT: factor a shuffle out of the inner loop and merge it into fft_permute.

6% faster SSE FFT on Conroe, 2.5% on Penryn.

Signed-off-by: Janne Grunau <janne-ffmpeg@jannau.net>
(cherry picked from commit e6b1ed693ae4098e6b9eabf938fc31ec0b09b120)
