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/parse.y

@@ -941,13 +941,10 @@ idlist(A) ::= nm(Y).
       p->pLeft = p->pRight = 0;
       p->x.pList = 0;
       p->pAggInfo = 0;
-      p->pTab = 0;
+      p->y.pTab = 0;
       p->op2 = 0;
       p->iTable = 0;
       p->iColumn = 0;
-#ifndef SQLITE_OMIT_WINDOWFUNC
-      p->pWin = 0;
-#endif
       p->u.zToken = (char*)&p[1];
       memcpy(p->u.zToken, t.z, t.n);
       p->u.zToken[t.n] = 0;