Demonstration code on a possible technique for optimizing the use of IN

operator on columns to the right of multicolumn indexes.  If the OP_Noop
generated where were really a new opcode that checked to see if there existed
any entries in the index with a matching prefix, it might prevent unnecessary
iterations of the IN operator.

FossilOrigin-Name: 92f0fe155d5546fc7f4a443d0630613dabe149061966308e5420fad652278f16

src/wherecode.c

@@ -592,6 +592,8 @@ static int codeEqualityTerm(
           if( i==iEq ){
             pIn->iCur = iTab;
             pIn->eEndLoopOp = bRev ? OP_PrevIfOpen : OP_NextIfOpen;
+            pIn->iBase = iReg - i;
+            pIn->nPrefix = i;
           }else{
             pIn->eEndLoopOp = OP_Noop;
           }