Bridge methods for embedded interfaces were
passing the interface twice: once as receiver,
once as first param.
Covered by $GOROOT/test/ddd.go.

Also:
- invent names ("arg%d") for parameters if missing.
- refactoring: move common code for bridge methods into
  createParams and emitTailCall.

R=gri
CC=golang-dev
https://golang.org/cl/7437047