image/jpeg: move the level-shift and clip out of the idct function,
to be consistent with the fdct function, and to ease any future
idct rewrites in assembly.
The BenchmarkIDCT delta is obviously just an accounting change and not
a real saving, but it does give an indication of what proportion of
time was spent in the actual IDCT and what proportion was in shift and
clip. The idct time taken is now comparable to fdct.
The BenchmarkFDCT delta is an estimate of benchmark noise.
benchmark old ns/op new ns/op delta
BenchmarkFDCT 3842 3837 -0.13%
BenchmarkIDCT 5611 3478 -38.01%
BenchmarkDecodeRGBOpaque
2932785 2929751 -0.10%
R=r
CC=golang-dev
https://golang.org/cl/
6625057