go.tools/go/exact: more consistent handling of unknown values
go.tools/go/exact: more consistent handling of unknown values

R=adonovan
CC=golang-dev
https://golang.org/cl/10431046

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