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