FFT: simplify fft8()

Part of the fft8() function corresponds to the BUTTERFLIES macro,
so use it.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/fft.c

@@ -246,21 +246,16 @@ static void fft4(FFTComplex *z)
 
 static void fft8(FFTComplex *z)
 {
-    FFTDouble t1, t2, t3, t4, t5, t6, t7, t8;
+    FFTDouble t1, t2, t3, t4, t5, t6;
 
     fft4(z);
 
     BF(t1, z[5].re, z[4].re, -z[5].re);
     BF(t2, z[5].im, z[4].im, -z[5].im);
-    BF(t3, z[7].re, z[6].re, -z[7].re);
-    BF(t4, z[7].im, z[6].im, -z[7].im);
-    BF(t8, t1, t3, t1);
-    BF(t7, t2, t2, t4);
-    BF(z[4].re, z[0].re, z[0].re, t1);
-    BF(z[4].im, z[0].im, z[0].im, t2);
-    BF(z[6].re, z[2].re, z[2].re, t7);
-    BF(z[6].im, z[2].im, z[2].im, t8);
+    BF(t5, z[7].re, z[6].re, -z[7].re);
+    BF(t6, z[7].im, z[6].im, -z[7].im);
 
+    BUTTERFLIES(z[0],z[2],z[4],z[6]);
     TRANSFORM(z[1],z[3],z[5],z[7],sqrthalf,sqrthalf);
 }