exp/ssa: fix bug in bridge method

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://codereview.appspot.com/7437047
exp/ssa/interp: uncomment tests now passing thanks to recent typechecker fixes.

Also: add support for pointer conversions, covered by one new test.

R=gri, bradfitz, dave
CC=golang-dev
https://codereview.appspot.com/7435047