Remove FF_MM_SSE2/3 flags for CPUs where this is generally not faster than

Remove FF_MM_SSE2/3 flags for CPUs where this is generally not faster than
regular MMX code. Examples of this are the Core1 CPU. Instead, set a new flag,
FF_MM_SSE2/3SLOW, which can be checked for particular SSE2/3 functions that
have been checked specifically on such CPUs and are actually faster than
their MMX counterparts.

In addition, use this flag to enable particular VP8 and LPC SSE2 functions
that are faster than their MMX counterparts.

Based on a patch by Loren Merritt <lorenm AT u washington edu>.

Originally committed as revision 24340 to svn://svn.ffmpeg.org/ffmpeg/trunk