Invalid behaviour in assignments with slice indexing on left side

[zapateo]

This code:

package main

import "fmt"

func main() {
	x := []int{1, 2, 3}
	i := 0
	i, x[i] = 1, 100
	fmt.Println(x)
}

should print:

[100 2 3]

but intestead it prints:

[1 100 3]

Note: this issue may be related to #534.