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/equalfuncs.c

@@ -1439,6 +1439,7 @@ _equalAlterEnumStmt(const AlterEnumStmt *a, const AlterEnumStmt *b)
 	COMPARE_STRING_FIELD(newVal);
 	COMPARE_STRING_FIELD(newValNeighbor);
 	COMPARE_SCALAR_FIELD(newValIsAfter);
+	COMPARE_SCALAR_FIELD(skipIfExists);
 
 	return true;
 }