regrename.c: Add general comment describing the pass.

regrename.c: Add general comment describing the pass.

	* regrename.c: Add general comment describing the pass.
	(struct du_head): Remove 'length' field.
	(get_element, merge_sort_comparison, merge, sort_du_head): Remove.
	(regrename_optimize): Do not sort chains.  Rework comments, add others.
	Force renaming to the preferred class (if any) in the first pass and do
	not consider registers that belong to it in the second pass.
	(create_new_chain): Do not set 'length' field.
	(scan_rtx_reg): Likewise.

From-SVN: r168404