PR rtl-opt/38582
	* regrename.c (struct du_head): New structure; some elements moved
	from...
	(struct du_chain): ... this one.
	(open_chains, closed_chains): Now of type struct du_head *.
	(do_replace): Accept du_head argument, not du_chain.  All callers
	changed.  Modified code to match new data structures.
	(build_def_use): Return a list of du_head structures.  Modified code
	to match new data structures.
	(dump_def_use_chain): Accept du_head argument, not du_chain.  All
	callers changed.  Modified code to match new data structures.
	(merge_overlapping_regs): Accept du_head argument, not du_chain.  All
	callers changed.  Modified code to match new data structures.
	(scan_rtx_reg): Change type of this_regno and this_nregs to unsigned.
	Allocate a du_head structure as well as a du_chain when creating a
	new chain.  Modified other code to match new data structures.

From-SVN: r154123