OpenMP/Fortran: Group handling of 'if' clause without and with modifier

The 'if' clause with modifier was introduced in
commit b4c3a85b (Subversion r242037)
"Partial OpenMP 4.5 fortran support", but -- in some instances -- didn't place
it next to the existing handling of 'if' clause without modifier.  Unify that;
no change in behavior.

	gcc/fortran/
	* dump-parse-tree.cc (show_omp_clauses): Group handling of 'if'
	clause without and with modifier.
	* frontend-passes.cc (gfc_code_walker): Likewise.
	* gfortran.h (gfc_omp_clauses): Likewise.
	* openmp.cc (gfc_free_omp_clauses): Likewise.