image/jpeg: move the huffman bit decoder state higher up in the

image/jpeg: move the huffman bit decoder state higher up in the
decoder struct, inside the unmappedzero limit, to eliminate some
TESTB instructions in the inner decoding loop.

benchmark          old ns/op    new ns/op    delta
BenchmarkDecode      2943204      2746360   -6.69%

R=r, dave
CC=golang-dev
https://golang.org/cl/6625058