use xorps instead of mulps to toggle the sign of a float, as suggested by...

use xorps instead of mulps to toggle the sign of a float, as suggested by Software Optimization Guide for AMD64 Processors.
Patch by Zuxy Meng < zuxy POIS meng AH  gmail POIS com > OKed by Michael
Original thread:
Date: Mar 5, 2006 8:15 PM
Subject: [Ffmpeg-devel] [PATCH] Little optimization to fft_sse.c

Originally committed as revision 5112 to svn://svn.ffmpeg.org/ffmpeg/trunk