Fix my recent prefetch changes:  - the first prefetch is actually harmful on Haswell with FMA,    but it is the most beneficial on ARM.  - the second prefetch... I was very stupid and multiplied by sizeof(scalar)    and offset of a scalar* pointer. The old offset was 64; pk = 8, so 64=pk*8.    So this effectively restores the older offset. Actually, there were    two prefetches here, one with offset 48 and one with offset 64. I could not    confirm any benefit from this strange 48 offset on either the haswell or    my ARM device.
Fix my recent prefetch changes:
 - the first prefetch is actually harmful on Haswell with FMA,
   but it is the most beneficial on ARM.
 - the second prefetch... I was very stupid and multiplied by sizeof(scalar)
   and offset of a scalar* pointer. The old offset was 64; pk = 8, so 64=pk*8.
   So this effectively restores the older offset. Actually, there were
   two prefetches here, one with offset 48 and one with offset 64. I could not
   confirm any benefit from this strange 48 offset on either the haswell or
   my ARM device.

mjbshaw/Eigen,madlib/eigen_backup,rotorliu/eigen,madlib/eigen_backup,rotorliu/eigen,madlib/eigen_backup,mjbshaw/Eigen,mjbshaw/Eigen,rotorliu/eigen,mjbshaw/Eigen,madlib/eigen_backup,rotorliu/eigen