Consistently order 'OMP_CLAUSE_SELF' right after 'OMP_CLAUSE_IF'

Consistently order 'OMP_CLAUSE_SELF' right after 'OMP_CLAUSE_IF'

As noted in recent commit 3a3596389c2e539cb8fd5dc5784a4e2afe193a2a
"OpenACC 2.7: Implement self clause for compute constructs", the OpenACC 'self'
clause very much relates to the 'if' clause, and therefore copies a lot of the
latter's handling.  Therefore it makes sense to also place this handling in
proximity to that of the 'if' clause, which was done in a lot but not all
instances.

	gcc/
	* tree-core.h (omp_clause_code): Move 'OMP_CLAUSE_SELF' after
	'OMP_CLAUSE_IF'.
	* tree-pretty-print.cc (dump_omp_clause): Adjust.
	* tree.cc (omp_clause_num_ops, omp_clause_code_name): Likewise.
	* tree.h: Likewise.