tree-loop-distribution.c: Add general explanantion on the pass.

tree-loop-distribution.c: Add general explanantion on the pass.

	* tree-loop-distribution.c: Add general explanantion on the pass.
	(generate_loops_for_partition): Mark distributed loop.
	(pg_add_dependence_edges): New parameter.  Handle alias data
	dependence specially and record it in the parameter if asked.
	(struct pg_vdata, pg_edata, pg_edge_callback_data): New structs.
	(init_partition_graph_vertices, add_partition_graph_edge): New.
	(pg_skip_alias_edge, free_partition_graph_edata_cb): New.
	(free_partition_graph_vdata, build_partition_graph): New.
	(sort_partitions_by_post_order, merge_dep_scc_partitions): New.
	(pg_collect_alias_ddrs, break_alias_scc_partitions): New.
	(data_ref_segment_size, latch_dominated_by_data_ref): New.
	(compute_alias_check_pairs, version_loop_by_alias_check): New.
	(version_for_distribution_p, finalize_partitions): New.
	(distribute_loop): Handle alias data dependence specially.  Factor
	out loop fusion code as functions and call these functions.

	gcc/testsuite
	* gcc.dg/tree-ssa/ldist-4.c: Adjust test string.
	* gcc.dg/tree-ssa/ldist-12.c: Ditto.
	* gcc.dg/tree-ssa/ldist-13.c: Ditto.
	* gcc.dg/tree-ssa/ldist-14.c: Ditto.

From-SVN: r249994