op_commutative.c: use ompi_op_is_commute(), not the expanded test

One more commit for this ticket... as pointed out by Giles, we have
ompi_op_is_commute().  We should use that instead of replicating the
logic for the test.

Refs trac:4548

This commit was SVN r31497.

The following Trac tickets were found above:
  Ticket 4548 --> https://svn.open-mpi.org/trac/ompi/ticket/4548

ompi/mpi/c/op_commutative.c

@@ -56,8 +56,8 @@ int MPI_Op_commutative(MPI_Op op, int *commute)
     }
 
     /* We have a valid op, get the flag */
-    
-    *commute = (op->o_flags & OMPI_OP_FLAGS_COMMUTE) ? 1 : 0;
+
+    *commute = ompi_op_is_commute(op);
 
     /* All done */