go.tools/go/types: fix StdSizes.Sizeof computation
go.tools/go/types: fix StdSizes.Sizeof computation

Thanks to Travis Cline for finding this bug.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/14752043

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old