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optimisation: reworked intra_prediction() so that yy is computed fully only
once and then updated for each iteration

--- a/libcelt/vq.c

+++ b/libcelt/vq.c

@@ -280,6 +280,7 @@

    celt_word32_t E;

    celt_word16_t pred_gain;

    int max_pos = N0-N;

+   celt_word32_t yy=0;

    VARDECL(celt_norm_t, Xr);

    SAVE_STACK;

@@ -293,21 +294,21 @@

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

          Xr[B*N-B*j-B+c] = x[B*j+c];

+   /* Compute yy for i=0 */

+   j=0;

+   do {

+      yy = MAC16_16(yy, Y[j], Y[j]);

+   } while (++j<B*N); /* Promises we loop at least once */

+

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

    {

-      celt_word32_t xy=0, yy=0;

-      celt_word16_t num;

-      celt_word16_t den;

+      celt_word32_t xy=0;

+      celt_word16_t num, den;

       const celt_word16_t * restrict xp = Xr;

       const celt_word16_t * restrict yp = Y+B*i;

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

-         complain to your compilor vendor */

       j=0;

       do {

-         xy = MAC16_16(xy, *xp, *yp);

-         yy = MAC16_16(yy, *yp, *yp);

-         xp++;

-         yp++;

+         xy = MAC16_16(xy, *xp++, *yp++);

       } while (++j<B*N); /* Promises we loop at least once */

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

       num = MULT16_16_Q15(ROUND16(xy,14),ROUND16(xy,14));

@@ -322,6 +323,13 @@

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

          s = ROUND16(xy,14);

       }

+      /* Update yy for the next iteration */

+      yp = Y+B*i;

+      j=0;

+      do {

+         yy = yy - MULT16_16(*yp, *yp) + MULT16_16(yp[B*N], yp[B*N]);

+         yp++;

+      } while (++j<B);

    }

    if (s<0)

    {