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
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go