go.tools/go/ssa/interp: minor cleanup
go.tools/go/ssa/interp: minor cleanup

LGTM=gri
R=gri
CC=golang-codereviews
https://codereview.appspot.com/88080044

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