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
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index 74438b5..ba24c07 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -2459,11 +2459,11 @@
                          ** TK_COLUMN: the value of p5 for OP_Column
                          ** TK_AGG_FUNCTION: nesting depth */
   AggInfo *pAggInfo;     /* Used by TK_AGG_COLUMN and TK_AGG_FUNCTION */
-  Table *pTab;           /* Table for TK_COLUMN expressions.  Can be NULL
-                         ** for a column of an index on an expression */
-#ifndef SQLITE_OMIT_WINDOWFUNC
-  Window *pWin;          /* Window definition for window functions */
-#endif
+  union {
+    Table *pTab;           /* TK_COLUMN: Table containing column. Can be NULL
+                           ** for a column of an index on an expression */
+    Window *pWin;          /* TK_FUNCTION: Window definition for the func */
+  } y;
 };
 
 /*
@@ -2493,6 +2493,7 @@
 #define EP_Subquery  0x200000 /* Tree contains a TK_SELECT operator */
 #define EP_Alias     0x400000 /* Is an alias for a result set column */
 #define EP_Leaf      0x800000 /* Expr.pLeft, .pRight, .u.pSelect all NULL */
+#define EP_WinFunc  0x1000000 /* TK_FUNCTION with Expr.y.pWin set */
 
 /*
 ** The EP_Propagate mask is a set of properties that automatically propagate