Combine the Expr.pTab and Expr.pWin fields into a union named "y". Add a new

EP_WinFunc property that is only true if Expr.y.pWin is a valid pointer.
This reduces the size of the Expr object by 8 bytes, reduces the overall
amount of code, and shaves over 1 million cycles off of the speed test.

FossilOrigin-Name: ad130bb86e74e6ce165fdbdce3a19699510f0e62071c1c7923b5a4538d888c7c

src/fkey.c

@@ -502,7 +502,7 @@ static Expr *exprTableColumn(
 ){
   Expr *pExpr = sqlite3Expr(db, TK_COLUMN, 0);
   if( pExpr ){
-    pExpr->pTab = pTab;
+    pExpr->y.pTab = pTab;
     pExpr->iTable = iCursor;
     pExpr->iColumn = iCol;
   }