rtl-ssa: Add entry point to allow re-parenting uses

This is needed by the upcoming aarch64 load pair pass, as it can
re-order stores (when alias analysis determines this is safe) and thus
change which mem def a given use consumes (in the RTL-SSA view, there is
no alias disambiguation of memory).

gcc/ChangeLog:

	* rtl-ssa/accesses.cc (function_info::reparent_use): New.
	* rtl-ssa/functions.h (function_info): Declare new member
	function reparent_use.