go.tools/go/types: return invalid type (rather than nil) for (*Label).Type()
go.tools/go/types: return invalid type (rather than nil) for (*Label).Type()

R=adonovan
CC=golang-dev
https://golang.org/cl/14782043

jdhenke/tools,dmitris/tools,sshetty10/tools,sshetty10/tools,nicot/tools,maddyonline/tools,syreclabs/go-tools,syreclabs/go-tools,springning/tools,runcom/tools,matryer/tools,jdhenke/tools,golang/tools,maddyonline/tools,magiccao/tools,cw2018/tools,syreclabs/go-tools,runcom/tools,glasser/golang-tools,cmars/tools,nicot/tools,4ad/tools,sshetty10/tools,josharian/tools,golang/tools,4ad/tools,syreclabs/go-tools,cw2018/tools,syreclabs/go-tools,syreclabs/go-tools,jdhenke/tools,jdhenke/tools,cw2018/tools,cmars/tools,cmars/tools,maddyonline/tools,glasser/golang-tools,dmitris/tools,argandas/tools,glasser/golang-tools,golang/tools,matryer/tools,golang/tools,josharian/tools,sshetty10/tools,dmitris/tools,magiccao/tools,magiccao/tools,matryer/tools,argandas/tools,springning/tools,springning/tools,golang/tools,runcom/tools,matryer/tools,runcom/tools,argandas/tools,dmitris/tools,magiccao/tools,nicot/tools,maddyonline/tools,argandas/tools,cmars/tools,cw2018/tools,4ad/tools,nicot/tools,springning/tools,glasser/golang-tools,josharian/tools,josharian/tools