SBR DSP: unroll sum_square

The length is even, so some unrolling can be performed. Timings are for x86:
- 32bits: 102c -> 82c
- 64bits:  82c -> 69c

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/sbrdsp.c

@@ -35,13 +35,18 @@ static void sbr_sum64x5_c(float *z)
 
 static float sbr_sum_square_c(float (*x)[2], int n)
 {
-    float sum = 0.0f;
+    float sum0 = 0.0f, sum1 = 0.0f;
     int i;
 
-    for (i = 0; i < n; i++)
-        sum += x[i][0] * x[i][0] + x[i][1] * x[i][1];
+    for (i = 0; i < n; i += 2)
+    {
+        sum0 += x[i + 0][0] * x[i + 0][0];
+        sum1 += x[i + 0][1] * x[i + 0][1];
+        sum0 += x[i + 1][0] * x[i + 1][0];
+        sum1 += x[i + 1][1] * x[i + 1][1];
+    }
 
-    return sum;
+    return sum0 + sum1;
 }
 
 static void sbr_neg_odd_64_c(float *x)