Refactor get_sign_bit_cost

Change-Id: I646e93abb1064d05bac625ba4c567d377b249367

av1/encoder/encodetxb.c

diff --git a/av1/encoder/encodetxb.c b/av1/encoder/encodetxb.c
index cac1aa5..3c447d5 100644
--- a/av1/encoder/encodetxb.c
+++ b/av1/encoder/encodetxb.c
@@ -243,9 +243,8 @@
 static INLINE int get_sign_bit_cost(tran_low_t qc, int coeff_idx,
                                     const int (*dc_sign_cost)[2],
                                     int dc_sign_ctx) {
-  const int sign = (qc < 0) ? 1 : 0;
-  // sign bit cost
   if (coeff_idx == 0) {
+    const int sign = (qc < 0) ? 1 : 0;
     return dc_sign_cost[dc_sign_ctx][sign];
   }
   return av1_cost_literal(1);