use shorter types vec_"type" instead of the too long vector "type"

part 1 of h264 luma interpolation 8x8 for altivec contributed by
Mauricio Alvarez % lokifo A gmail P com %
Original thread:
Date: Jun 26, 2007 8:15 PM
Subject: Re: [FFmpeg-devel] [PATCH] h264 luma interpolation 8x8 for altivec

Originally committed as revision 10090 to svn://svn.ffmpeg.org/ffmpeg/trunk