cmd/gc: refactor racewalk It is refactoring towards generic walk + it handles mode nodes. Partially fixes 4228 issue.
cmd/gc: refactor racewalk
It is refactoring towards generic walk
+ it handles mode nodes.
Partially fixes 4228 issue.

R=golang-dev, lvd, rsc
CC=golang-dev
http://codereview.appspot.com/6775098

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