re-write 8 point idct

to be consistent with idct16 and idct32.

Change-Id: Ie89dbd32b65c33274b7fecb4b41160fcf1962204

vp9/common/vp9_rtcd_defs.sh

@@ -389,6 +389,9 @@ specialize vp9_short_idct8x8
 prototype void vp9_short_idct10_8x8 "int16_t *input, int16_t *output, int pitch"
 specialize vp9_short_idct10_8x8
 
+prototype void vp9_short_idct1_8x8 "int16_t *input, int16_t *output"
+specialize vp9_short_idct1_8x8
+
 prototype void vp9_short_ihaar2x2 "int16_t *input, int16_t *output, int pitch"
 specialize vp9_short_ihaar2x2
 

vp9/decoder/vp9_dequantize.c

@@ -212,14 +212,11 @@ void vp9_dequant_idct_add_8x8_c(int16_t *input, const int16_t *dq,
     vp9_copy_mem8x8(pred, pitch, dest, stride);
   } else if (eob == 1) {
     /* DC only DCT coefficient. */
+    int16_t in = input[0];
     int16_t out;
-
     /* Note: the idct1 will need to be modified accordingly whenever
      * vp9_short_idct8x8_c() is modified. */
-    out = (input[0] + 1 + (input[0] < 0)) >> 2;
-    out = out << 3;
-    out = (out + 32) >> 7;
-
+    vp9_short_idct1_8x8_c(&in, &out);
     input[0] = 0;
 
     add_constant_residual(out, pred, pitch, dest, stride, 8, 8);