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

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

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@5112 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b