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

tv42/old-go,dsriram/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,tv42/old-go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,tv42/old-go,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme