go.tools/go/types: use WordSize as function pointer size
go.tools/go/types: use WordSize as function pointer size

Both gc and gccgo appear to use this size, so it seems appropriate
to use it here.

R=gri
CC=golang-codereviews, golang-dev, iant, rsc
https://golang.org/cl/52290043

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