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/treeview.c

@@ -497,7 +497,7 @@ void sqlite3TreeViewExpr(TreeView *pView, const Expr *pExpr, u8 moreToFollow){
       }else{
         pFarg = pExpr->x.pList;
 #ifndef SQLITE_OMIT_WINDOWFUNC
-        pWin = pExpr->pWin;
+        pWin = pExpr->y.pWin;
 #else
         pWin = 0;
 #endif