ppc: blend_h pwr9 implementation

blend_h_w2_8bpc_pwr9:       18.4 ( 1.20x)
blend_h_w4_8bpc_pwr9:       27.2 ( 1.26x)
blend_h_w8_8bpc_pwr9:       27.9 ( 2.22x)
blend_h_w16_8bpc_pwr9:      35.1 ( 3.28x)
blend_h_w32_8bpc_pwr9:      57.4 ( 3.88x)
blend_h_w64_8bpc_pwr9:      97.9 ( 4.70x)
blend_h_w128_8bpc_pwr9:    207.6 ( 5.18x)