cmd/gc: warn about slice indexes larger than int in typecheck pass
cmd/gc: warn about slice indexes larger than int in typecheck pass

Fixes GOARCH=386 build.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/6810098

firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old