go/tools/ssa/interp: make iface.eq robust against nil types
go/tools/ssa/interp: make iface.eq robust against nil types

A future version of go/types.IsIdentical does not accept
nil types anymore.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/16150043

chenjianlong/go.tools,karmakaze/go.tools,lattera/go.tools,chenjianlong/go.tools,chenjianlong/go.tools,karmakaze/go.tools,lattera/go.tools,karmakaze/go.tools,karmakaze/go.tools,lattera/go.tools,karmakaze/go.tools