duplicate code emitted for type aliases

[zeldovich]

% cat foo.go 
package main

//go:generate msgp -tests=false -io=false

type A int
type B = A

func main() {
}
% go generate .
======== MessagePack Code Generator =======
>>> Input: "foo.go"
>>> Wrote and formatted "foo_gen.go"
% go build .
./foo_gen.go:38:12: A.MarshalMsg redeclared in this block
	previous declaration at ./foo_gen.go:10:6
./foo_gen.go:45:13: (*A).UnmarshalMsg redeclared in this block
	previous declaration at ./foo_gen.go:17:6
./foo_gen.go:60:12: A.Msgsize redeclared in this block
	previous declaration at ./foo_gen.go:32:6
% go version
go version go1.12 linux/amd64
%