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optimisation: intra_prediction() no longer needs to divide inside the search

--- a/libcelt/vq.c

+++ b/libcelt/vq.c

@@ -266,7 +266,8 @@

 {

    int i,j;

    int best=0;

-   celt_word32_t best_score=0;

+   celt_word32_t best_num=-SHR32(VERY_LARGE32,4);

+   celt_word16_t best_den=0;

    celt_word16_t s = 1;

    int sign;

    celt_word32_t E;

@@ -278,7 +279,8 @@

    for (i=0;i<max_pos*B;i+=B)

    {

       celt_word32_t xy=0, yy=0;

-      celt_word32_t score;

+      celt_word32_t num;

+      celt_word16_t den;

       /* If this doesn't generate a double-MAC on supported architectures, 

          complain to your compilor vendor */

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

@@ -286,11 +288,14 @@

          xy = MAC16_16(xy, x[j], Y[i+N-j-1]);

          yy = MAC16_16(yy, Y[i+N-j-1], Y[i+N-j-1]);

       }

+      /* Using xy^2/yy as the score but without having to do the division */

+      num = MULT16_16(ROUND16(xy,14),ROUND16(xy,14));

+      den = ROUND16(yy,14);

       /* If you're really desperate for speed, just use xy as the score */

-      score = celt_div(MULT16_16(ROUND16(xy,14),ROUND16(xy,14)), ROUND16(yy,14));

-      if (score > best_score)

+      if (MULT16_32_Q15(best_den, num) >  MULT16_32_Q15(den, best_num))

       {

-         best_score = score;

+         best_num = num;

+         best_den = den;

          best = i;

          /* Store xy as the sign. We'll normalise it to +/- 1 later. */

          s = ROUND16(xy,14);