gccgo: mishandles iota in len([iota]int{}) expression

https://play.golang.org/p/ZBwHogxkJp

    package main

    const (
            m = len([iota]int{})
            n
            o
    )

    func main() {
            println(m, n, o)
    }

prints "0 1 2" with cmd/compile, but "0 0 0" with gccgo.

/cc @ianlancetaylor 