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/backend/nodes/copyfuncs.c

@@ -3055,6 +3055,7 @@ _copyAlterEnumStmt(const AlterEnumStmt *from)
 	COPY_STRING_FIELD(newVal);
 	COPY_STRING_FIELD(newValNeighbor);
 	COPY_SCALAR_FIELD(newValIsAfter);
+	COPY_SCALAR_FIELD(skipIfExists);
 
 	return newnode;
 }