rtl-ssa: Provide easier access to debug uses [PR113089]

This patch adds some accessors to set_info and use_info to make it
easier to get at and iterate through uses in debug insns.

It is used by the aarch64 load/store pair fusion pass in a subsequent
patch to fix PR113089, i.e. to update debug uses in the pass.

gcc/ChangeLog:

	PR target/113089
	* rtl-ssa/accesses.h (use_info::next_debug_insn_use): New.
	(debug_insn_use_iterator): New.
	(set_info::first_debug_insn_use): New.
	(set_info::debug_insn_uses): New.
	* rtl-ssa/member-fns.inl (use_info::next_debug_insn_use): New.
	(set_info::first_debug_insn_use): New.
	(set_info::debug_insn_uses): New.