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Reducing dependencies in deemphasis()

Reordering the add with VERY_SMALL changes the dependencies cycle from 2 add + 1 mul
(11 cycles on haswell) to 1 add + 1 mul (8 cycles). This makes the entire decoder about
1.5% faster.

--- a/celt/celt_decoder.c

+++ b/celt/celt_decoder.c

@@ -225,7 +225,7 @@

          /* Shortcut for the standard (non-custom modes) case */

          for (j=0;j<N;j++)

          {

-            celt_sig tmp = x[j] + m + VERY_SMALL;

+            celt_sig tmp = x[j] + VERY_SMALL + m;

             m = MULT16_32_Q15(coef0, tmp);

             scratch[j] = tmp;

          }

@@ -246,7 +246,7 @@

          {

             for (j=0;j<N;j++)

             {

-               celt_sig tmp = x[j] + m + VERY_SMALL;

+               celt_sig tmp = x[j] + VERY_SMALL + m;

                m = MULT16_32_Q15(coef0, tmp);

                y[j*C] = SCALEOUT(SIG2WORD16(tmp));

             }