Allow IF NOT EXISTS when add a new enum label.

If the label is already in the enum the statement becomes a no-op.
This will reduce the pain that comes from our not allowing this
operation inside a transaction block.

Andrew Dunstan, reviewed by Tom Lane and Magnus Hagander.

src/include/nodes/parsenodes.h

@@ -2306,6 +2306,7 @@ typedef struct AlterEnumStmt
 	char	   *newVal;			/* new enum value's name */
 	char	   *newValNeighbor; /* neighboring enum value, if specified */
 	bool		newValIsAfter;	/* place new enum value after neighbor? */
+	bool        skipIfExists;   /* ignore statement if label already exists */
 } AlterEnumStmt;
 
 /* ----------------------