Make all gimple_omp_for_ accessors typesafe

Make all gimple_omp_for_ accessors typesafe

gcc/ChangeLog.gimple-classes:
	* gimple.h (gimple_omp_for_kind): Strengthen param from
	const_gimple to const gomp_for *.
	(gimple_omp_for_combined_p): Likewise.
	(gimple_omp_for_combined_into_p): Likewise.
	(gimple_omp_for_clauses): Likewise.
	(gimple_omp_for_index): Likewise.
	(gimple_omp_for_initial): Likewise.
	(gimple_omp_for_final): Likewise.
	(gimple_omp_for_incr): Likewise.
	(gimple_omp_for_pre_body): Likewise.
	(gimple_omp_for_cond): Likewise.
	(gimple_omp_for_clauses_ptr): Strengthen param from gimple to
	gomp_for *.
	(gimple_omp_for_set_clauses): Likewise.
	(gimple_omp_for_index_ptr): Likewise.
	(gimple_omp_for_set_index): Likewise.
	(gimple_omp_for_initial_ptr): Likewise.
	(gimple_omp_for_set_initial): Likewise.
	(gimple_omp_for_final_ptr): Likewise.
	(gimple_omp_for_set_final): Likewise.
	(gimple_omp_for_incr_ptr): Likewise.
	(gimple_omp_for_set_incr): Likewise.
	(gimple_omp_for_pre_body_ptr): Likewise.
	(gimple_omp_for_set_pre_body): Likewise.
	(gimple_omp_for_set_cond): Likewise.
	(gimple_omp_for_collapse): Strengthen param from gimple to
	const gomp_for *.

	* gimple-walk.c (walk_gimple_op): Within case GIMPLE_OMP_FOR,
	introduce local "omp_for_stmt" via a checked cast and use it in
	place of "stmt" for typesafety.
	(walk_gimple_stmt): Add checked cast.
	* gimple.c (gimple_copy): Within case GIMPLE_OMP_FOR, introduce
	locals "omp_for_stmt" and "omp_for_copy" via checked casts and use
	them in place of "stmt" and "copy" for typesafety.
	* omp-low.c (determine_parallel_type): Add a checked cast.
	(build_outer_var_ref): Likewise.
	(find_combined_for): Within case GIMPLE_OMP_FOR,
	introduce local "omp_for_stmt" via a checked cast and use it in
	place of "stmt" for typesafety.
	(check_omp_nesting_restrictions): Add checked casts.
	(check_omp_nesting_restrictions): Likewise.  Within
	case GIMPLE_OMP_FOR, introduce local "omp_for_stmt" via a checked
	cast and use it in place of "stmt" for typesafety.
	(scan_omp_1_stmt): Add checked cast.
	(lower_rec_simd_input_clauses): Likewise.
	(lower_rec_input_clauses): Likewise.  Introduce two locals named
	"omp_for_ctx_stmt" via checked casts and use them in place of
	"ctx->stmt" for typesafety.
	(lower_lastprivate_clauses): Add checked cast.
	(lower_reduction_clauses): Likewise.
	(expand_omp_for_init_vars): Likewise.
	(expand_omp_for_generic): Introduce local "omp_for_inner_stmt" via
	a checked cast and use in place of "inner_stmt" for typesafety.
	(expand_omp_for_static_nochunk): Add checked cast.
	(expand_omp_for_static_chunk): Likewise.
	(expand_omp_for): Introduce local "omp_for_stmt" via a checked
	cast and use in place of "last_stmt (region->entry)" for
	typesafety.
	(expand_omp): Add a checked cast.
	(diagnose_sb_0): Add checked casts.
	(diagnose_sb_1): Within	case GIMPLE_OMP_FOR, introduce local
	"omp_for_stmt" via a checked cast and use it in place of "stmt" for
	typesafety.
	(diagnose_sb_2): Likewise.
	* tree-inline.c (remap_gimple_stmt): Likewise, introducing locals
	"omp_for_stmt" and "omp_for_copy", using in place of "stmt" and
	"copy".
	(estimate_num_insns): Likewise, introducing local "omp_for_stmt"
	for use in place of "stmt".
	* tree-nested.c (convert_nonlocal_reference_stmt): Likewise.
	(convert_local_reference_stmt): Likewise.
	(convert_gimple_call): Add a checked cast.