image/png: use branch-free abs function
benchmark old ns/op new ns/op delta
BenchmarkPaeth 5.06 6.02 +18.97%
BenchmarkDecodeGray
1010551 956911 -5.31%
BenchmarkDecodeNRGBAGradient
3877813 3754160 -3.19%
BenchmarkDecodeNRGBAOpaque
3194058 3079094 -3.60%
BenchmarkDecodePaletted 699243 700211 +0.14%
BenchmarkDecodeRGB
2835733 2692120 -5.06%
BenchmarkDecodeInterlacing
3651805 3563124 -2.43%
BenchmarkEncodeGray
4399183 4404113 +0.11%
BenchmarkEncodeNRGBOpaque
13323627 13306485 -0.13%
BenchmarkEncodeNRGBA
15840092 15751188 -0.56%
BenchmarkEncodePaletted
4396622 4404373 +0.18%
BenchmarkEncodeRGBOpaque
13320475 13279189 -0.31%
BenchmarkEncodeRGBA
36898392 36781002 -0.32%
LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://golang.org/cl/
117290043