Paletted, RGBA.

benchmark                       old ns/op    new ns/op    delta
BenchmarkDecodeGray               3681144      2536049  -31.11%
BenchmarkDecodeNRGBAGradient     12108660     10020650  -17.24%
BenchmarkDecodeNRGBAOpaque       10699230      8677165  -18.90%
BenchmarkDecodePaletted           2562806      1458798  -43.08%
BenchmarkDecodeRGB                8468175      7180730  -15.20%

benchmark                        old MB/s     new MB/s  speedup
BenchmarkDecodeGray                 17.80        25.84    1.45x
BenchmarkDecodeNRGBAGradient        21.65        26.16    1.21x
BenchmarkDecodeNRGBAOpaque          24.50        30.21    1.23x
BenchmarkDecodePaletted             25.57        44.92    1.76x
BenchmarkDecodeRGB                  30.96        36.51    1.18x

$ file $GOROOT/src/pkg/image/png/testdata/bench*
benchGray.png:           PNG image, 256 x 256, 8-bit grayscale, non-interlaced
benchNRGBA-gradient.png: PNG image, 256 x 256, 8-bit/color RGBA, non-interlaced
benchNRGBA-opaque.png:   PNG image, 256 x 256, 8-bit/color RGBA, non-interlaced
benchPaletted.png:       PNG image, 256 x 256, 8-bit colormap, non-interlaced
benchRGB.png:            PNG image, 256 x 256, 8-bit/color RGB, non-interlaced

R=r
CC=golang-dev
https://golang.org/cl/6127051