go.tools/go/types: cache method set for pointer-to-named in the Named.
go.tools/go/types: cache method set for pointer-to-named in the Named.

This change improves the complete running time of 'ssadump cmd/oracle' by ~20%.

R=gri, gri
CC=golang-dev
https://codereview.appspot.com/22910045

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