cabac_functions: Count zeros with ctz if it is fast

When refilling the low bit buffer after get_cabac count the bits with ctz
if the processor has a fast version.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cabac_functions.h

@@ -76,9 +76,12 @@ static inline void renorm_cabac_decoder_once(CABACContext *c){
 static void refill2(CABACContext *c){
     int i;
     unsigned x;
-
+#if !HAVE_FAST_CLZ
     x= c->low ^ (c->low-1);
     i= 7 - ff_h264_norm_shift[x>>(CABAC_BITS-1)];
+#else
+    i = ff_ctz(c->low) - CABAC_BITS;
+#endif
 
     x= -CABAC_MASK;