Alexandra Hájková authored Jul 11, 2018 and Henrik Gramner committed Aug 06, 2018

1) using xxpermdi + merge instead of 2 merges improves quant_8x8
performance by 5%

2) use vec_splats instead of vec_splat

checkasm timings when compiled with gcc:
                  C:            AltiVec:
                                before: after:
quant_2x2_dc:      57            163      46
quant_4x4_dc:     141            162      57

dequant_4x4_cmp:  104            101      45
dequant_4x4_flat: 104            106      46
dequant_8x8_cmp:  412            208     147
dequant_8x8_flat: 414            212     149