runtime/race: Race detector does not detect len(c) / {send,recv}(c) race

The race detector should report a race when one goroutine is reading the length of a channel and another is reading or writing to the channel.

``` go
func main() {
    res := make(chan int, 1)
    ch := make(chan bool, 1)
    go func() { ch <- true }()
    go func() { res <- len(ch) }()
    fmt.Println(<-res)
}
```

@dvyukov 
