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

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