cmd/cc: fix uint right shift in constant evaluation         Fixes issue 3664.
cmd/cc: fix uint right shift in constant evaluation
        Fixes issue 3664.

R=golang-dev, bradfitz, rsc
CC=golang-dev
http://codereview.appspot.com/6249048

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