test: add test for order of evaluation of map index on left of =

Gccgo used to get this wrong.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6121044

test/reorder.go

@@ -19,6 +19,7 @@ func main() {
 	p6()
 	p7()
 	p8()
+	p9()
 }
 
 var gx []int
@@ -119,3 +120,11 @@ func p8() {
 	i := 0
 	i, x[i], x[5] = 1, 100, 500
 }
+
+func p9() {
+	m := make(map[int]int)
+	m[0] = len(m)
+	if m[0] != 0 {
+		panic(m[0])
+	}
+}