cmd/gc: clarify a TODO regarding variables used in liveness analysis
cmd/gc: clarify a TODO regarding variables used in liveness analysis

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/38640043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang