rtl-ssa: Support inferring uses of mem in change_insns

Currently, rtl_ssa::change_insns requires all new uses and defs to be
specified explicitly.  This turns out to be rather inconvenient for
forming load pairs in the new aarch64 load pair pass, as the pass has to
determine which mem def the final load pair consumes, and then obtain or
create a suitable use (i.e. significant bookkeeping, just to keep the
RTL-SSA IR consistent).  It turns out to be much more convenient to
allow change_insns to infer which def is consumed and create a suitable
use of mem itself.  This patch does that.

gcc/ChangeLog:

	* rtl-ssa/changes.cc (function_info::finalize_new_accesses): Add new
	parameter to give final insn position, infer use of mem if it isn't
	specified explicitly.
	(function_info::change_insns): Pass down final insn position to
	finalize_new_accesses.
	* rtl-ssa/functions.h: Add parameter to finalize_new_accesses.