PR debug/77844
	* valtrack.c: Include rtl-iter.h.
	(struct rtx_subst_pair): Add insn field.
	(propagate_for_debug_subst): If pair->to contains at least 2
	regs, create a DEBUG_INSN with a debug temp before pair->insn
	and replace from with the debug temp instead of pair->to.
	(propagate_for_debug): Initialize p.insn.
	* combine.c (insn_uid_check): New inline function.
	(INSN_COST, LOG_LINKS): Use it instead of INSN_UID.
	(find_single_use, combine_instructions,
	cant_combine_insn_p, try_combine): Use NONDEBUG_INSN_P instead of
	INSN_P.

	* g++.dg/opt/pr77844.C: New test.

From-SVN: r243662