Use range-based for loops for traversing loops

Use range-based for loops for traversing loops

This patch follows Martin's suggestion here[1], to support
range based loop for iterating loops, analogously to the
patch for vec[2].

For example, use below range-based for loop

    for (auto loop : loops_list (cfun, 0))

to replace the previous macro FOR_EACH_LOOP

    FOR_EACH_LOOP (loop, 0)

[1] https://gcc.gnu.org/pipermail/gcc-patches/2021-June/573424.html
[2] https://gcc.gnu.org/pipermail/gcc-patches/2021-June/572315.html

gcc/ChangeLog:

	* cfgloop.h (as_const): New function.
	(class loop_iterator): Rename to ...
	(class loops_list): ... this.
	(loop_iterator::next): Rename to ...
	(loops_list::Iter::fill_curr_loop): ... this and adjust.
	(loop_iterator::loop_iterator): Rename to ...
	(loops_list::loops_list): ... this and adjust.
	(loops_list::Iter): New class.
	(loops_list::iterator): New type.
	(loops_list::const_iterator): New type.
	(loops_list::begin): New function.
	(loops_list::end): Likewise.
	(loops_list::begin const): Likewise.
	(loops_list::end const): Likewise.
	(FOR_EACH_LOOP): Remove.
	(FOR_EACH_LOOP_FN): Remove.
	* cfgloop.c (flow_loops_dump): Adjust FOR_EACH_LOOP* with range-based
	for loop with loops_list instance.
	(sort_sibling_loops): Likewise.
	(disambiguate_loops_with_multiple_latches): Likewise.
	(verify_loop_structure): Likewise.
	* cfgloopmanip.c (create_preheaders): Likewise.
	(force_single_succ_latches): Likewise.
	* config/aarch64/falkor-tag-collision-avoidance.c
	(execute_tag_collision_avoidance): Likewise.
	* config/mn10300/mn10300.c (mn10300_scan_for_setlb_lcc): Likewise.
	* config/s390/s390.c (s390_adjust_loops): Likewise.
	* doc/loop.texi: Likewise.
	* gimple-loop-interchange.cc (pass_linterchange::execute): Likewise.
	* gimple-loop-jam.c (tree_loop_unroll_and_jam): Likewise.
	* gimple-loop-versioning.cc (loop_versioning::analyze_blocks): Likewise.
	(loop_versioning::make_versioning_decisions): Likewise.
	* gimple-ssa-split-paths.c (split_paths): Likewise.
	* graphite-isl-ast-to-gimple.c (graphite_regenerate_ast_isl): Likewise.
	* graphite.c (canonicalize_loop_form): Likewise.
	(graphite_transform_loops): Likewise.
	* ipa-fnsummary.c (analyze_function_body): Likewise.
	* ipa-pure-const.c (analyze_function): Likewise.
	* loop-doloop.c (doloop_optimize_loops): Likewise.
	* loop-init.c (loop_optimizer_finalize): Likewise.
	(fix_loop_structure): Likewise.
	* loop-invariant.c (calculate_loop_reg_pressure): Likewise.
	(move_loop_invariants): Likewise.
	* loop-unroll.c (decide_unrolling): Likewise.
	(unroll_loops): Likewise.
	* modulo-sched.c (sms_schedule): Likewise.
	* predict.c (predict_loops): Likewise.
	(pass_profile::execute): Likewise.
	* profile.c (branch_prob): Likewise.
	* sel-sched-ir.c (sel_finish_pipelining): Likewise.
	(sel_find_rgns): Likewise.
	* tree-cfg.c (replace_loop_annotate): Likewise.
	(replace_uses_by): Likewise.
	(move_sese_region_to_fn): Likewise.
	* tree-if-conv.c (pass_if_conversion::execute): Likewise.
	* tree-loop-distribution.c (loop_distribution::execute): Likewise.
	* tree-parloops.c (parallelize_loops): Likewise.
	* tree-predcom.c (tree_predictive_commoning): Likewise.
	* tree-scalar-evolution.c (scev_initialize): Likewise.
	(scev_reset): Likewise.
	* tree-ssa-dce.c (find_obviously_necessary_stmts): Likewise.
	* tree-ssa-live.c (remove_unused_locals): Likewise.
	* tree-ssa-loop-ch.c (ch_base::copy_headers): Likewise.
	* tree-ssa-loop-im.c (analyze_memory_references): Likewise.
	(tree_ssa_lim_initialize): Likewise.
	* tree-ssa-loop-ivcanon.c (canonicalize_induction_variables): Likewise.
	* tree-ssa-loop-ivopts.c (tree_ssa_iv_optimize): Likewise.
	* tree-ssa-loop-manip.c (get_loops_exits): Likewise.
	* tree-ssa-loop-niter.c (estimate_numbers_of_iterations): Likewise.
	(free_numbers_of_iterations_estimates): Likewise.
	* tree-ssa-loop-prefetch.c (tree_ssa_prefetch_arrays): Likewise.
	* tree-ssa-loop-split.c (tree_ssa_split_loops): Likewise.
	* tree-ssa-loop-unswitch.c (tree_ssa_unswitch_loops): Likewise.
	* tree-ssa-loop.c (gate_oacc_kernels): Likewise.
	(pass_scev_cprop::execute): Likewise.
	* tree-ssa-propagate.c (clean_up_loop_closed_phi): Likewise.
	* tree-ssa-sccvn.c (do_rpo_vn): Likewise.
	* tree-ssa-threadupdate.c
	(jump_thread_path_registry::thread_through_all_blocks): Likewise.
	* tree-vectorizer.c (vectorize_loops): Likewise.
	* tree-vrp.c (vrp_asserts::find_assert_locations): Likewise.