Use remove some magic numbers in aom_rans_merge_prob8_pdf.

Change-Id: I0cefae17642d7adf1b9bd637ecb81b437629aa0c

aom_dsp/ans.c

diff --git a/aom_dsp/ans.c b/aom_dsp/ans.c
index 30f115c..6d705cd 100644
--- a/aom_dsp/ans.c
+++ b/aom_dsp/ans.c
@@ -39,7 +39,7 @@
   const int out_syms = in_syms + 1;
   assert(src_pdf != out_pdf);
 
-  out_pdf[0] = node_prob << (10 - 8);
+  out_pdf[0] = node_prob << (RANS_PROB_BITS - ANS_P8_SHIFT);
   adjustment -= out_pdf[0];
   for (i = 0; i < in_syms; ++i) {
     int p = (p1 * src_pdf[i] + round_fact) >> ANS_P8_SHIFT;