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.
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
http://codereview.appspot.com/6625058

rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,mhennings/marcohennings-go,scirelli/scirelli-go,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,bryanxu/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,scirelli/scirelli-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,bryanxu/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,webfd/go-zh,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,rflanagan/reginaldflanagan-project1,webfd/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,bryanxu/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,d0f/go-zh,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,webfd/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,scirelli/scirelli-go