runtime: fix complex division benchmarks

we can't add the division result to n during iteration, because it might
turn n into NaN or Inf.

R=golang-dev, rsc, iant, iant
CC=golang-dev
https://golang.org/cl/6197045

src/pkg/runtime/complex_test.go

@@ -14,44 +14,54 @@ var result complex128
 func BenchmarkComplex128DivNormal(b *testing.B) {
 	d := 15 + 2i
 	n := 32 + 3i
+	res := 0i
 	for i := 0; i < b.N; i++ {
-		n += n / d
+		n += 0.1i
+		res += n / d
 	}
-	result = n
+	result = res
 }
 
 func BenchmarkComplex128DivNisNaN(b *testing.B) {
 	d := cmplx.NaN()
 	n := 32 + 3i
+	res := 0i
 	for i := 0; i < b.N; i++ {
-		n += n / d
+		n += 0.1i
+		res += n / d
 	}
-	result = n
+	result = res
 }
 
 func BenchmarkComplex128DivDisNaN(b *testing.B) {
 	d := 15 + 2i
 	n := cmplx.NaN()
+	res := 0i
 	for i := 0; i < b.N; i++ {
-		n += n / d
+		d += 0.1i
+		res += n / d
 	}
-	result = n
+	result = res
 }
 
 func BenchmarkComplex128DivNisInf(b *testing.B) {
 	d := 15 + 2i
 	n := cmplx.Inf()
+	res := 0i
 	for i := 0; i < b.N; i++ {
-		n += n / d
+		d += 0.1i
+		res += n / d
 	}
-	result = n
+	result = res
 }
 
 func BenchmarkComplex128DivDisInf(b *testing.B) {
 	d := cmplx.Inf()
 	n := 32 + 3i
+	res := 0i
 	for i := 0; i < b.N; i++ {
-		n += n / d
+		n += 0.1i
+		res += n / d
 	}
-	result = n
+	result = res
 }