ppc: reduce overreads when loading 8 pixels in altivec dsp functions

Altivec can only load naturally aligned vectors. To handle possibly
unaligned data a second vector is loaded from an offset of the original
location and the data is recovered through a vector permutation.
Overreads are minimal if the offset for second load points to the last
element of data. This is 7 for loading eight 8-bit pixels and overreads
are reduced from 16 bytes to 8 bytes if the pixels are 64-bit aligned.
For unaligned pixels the overread is reduced from 23 bytes to 15 bytes
in the worst case.