go/types, types2: handle embedding of type parameters correctly

```Go
package p

func _[E1 interface{ m() E1 }, E2 interface{ E1 } ](c1 chan E1, c2 chan E2) {
        x1 := (<-c1).m()
        x2 := (<-c2).m()
        _ = x1
        _ = x2
}
```
produces
```
(<-c2).m undefined (type bound for E2 has no method m)
```
This should either be permitted or embedding of `E1` in the `E2` constraint should be explicitly forbidden.

cc: @findleyr 