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Further optimization to cwrsi()

Makes it possible to skip the first loop in some cases.

--- a/celt/cwrs.c

+++ b/celt/cwrs.c

@@ -461,11 +461,11 @@

 }

 static void cwrsi(int _n,int _k,opus_uint32 _i,int *_y){

+  int s;

   celt_assert(_k>0);

-  celt_assert(_n>0);

+  celt_assert(_n>1);

   do{

     opus_uint32 p;

-    int         s;

     int         k0;

     /*Are the pulses in this dimension negative?*/

     p=CELT_PVQ_U(_n,_k+1);

@@ -476,14 +476,29 @@

     p=CELT_PVQ_U(_n,_k);

     if(_k>_n){

       const opus_uint32 *row;

+      opus_uint32        q;

       row=CELT_PVQ_U_ROW[_n];

-      for(;p>_i&&_k>_n;p=row[_k])_k--;

+      q=row[_n];

+      if(q>_i){

+        celt_assert(p>q);

+        /*Setting p=q is unnecessary, but it helps the optimizer prove p>_i,

+           allowing it to jump straight past the initial test in the second

+           loop below.

+          Once it's removed that first comparison, a smart compiler should be

+           able to figure out that the result of this assignment isn't used and

+           optimize it away anyway.*/

+        p=q;

+        _k=_n;

+      }

+      else for(;p>_i;p=row[_k])_k--;

     }

     for(;p>_i;p=CELT_PVQ_U_ROW[_k][_n])_k--;

     _i-=p;

     *_y++=(k0-_k+s)^s;

   }

-  while(--_n>0);

+  while(--_n>1);

+  s=-(_i>=1);

+  *_y=(_k+s)^s;

 }

 void decode_pulses(int *_y,int _n,int _k,ec_dec *_dec){