op_commutative.c: don't check if the op is predefined or not

MPI_OP_COMMUTATIVE should work on all MPI_Op's -- regardless of
whether they are predefined or not.

Refs #4548.
op_commutative.c: ...but we should check for MPI_OP_NULL

Refs #4548
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 #4548