Optimize altivec instruction vpkpx (Vector Pack Pixel).
Rearranges 8 pixels coded in 6-5-5 pattern (4 from each source register)
into contigous array of bits in the destination register.
In each iteration of outer loop, the instruction is to be done with
the 6-5-5 pack for 2 pixels of each doublew
Optimize altivec instruction vpkpx (Vector Pack Pixel).
Rearranges 8 pixels coded in 6-5-5 pattern (4 from each source register)
into contigous array of bits in the destination register.
In each iteration of outer loop we do the 6-5-5 pack for 2 pixels
of each doubleword element of each source reg
