rtl-ssa: Support for inserting new insns

The upcoming aarch64 load pair pass needs to form store pairs, and can
re-order stores over loads when alias analysis determines this is safe.
In the case that both mem defs have uses in the RTL-SSA IR, and both
stores require re-ordering over their uses, we represent that as
(tentative) deletion of the original store insns and creation of a new
insn, to prevent requiring repeated re-parenting of uses during the
pass.  We then update all mem uses that require re-parenting in one go
at the end of the pass.

To support this, RTL-SSA needs to handle inserting new insns (rather
than just changing existing ones), so this patch adds support for that.

New insns (and new accesses) are temporaries, allocated above a temporary
obstack_watermark, such that the user can easily back out of a change without
awkward bookkeeping.

gcc/ChangeLog:

	* rtl-ssa/accesses.cc (function_info::create_set): New.
	* rtl-ssa/accesses.h (access_info::is_temporary): New.
	* rtl-ssa/changes.cc (move_insn): Handle new (temporary) insns.
	(function_info::finalize_new_accesses): Handle new/temporary
	user-created accesses.
	(function_info::apply_changes_to_insn): Ensure m_is_temp flag
	on new insns gets cleared.
	(function_info::change_insns): Handle new/temporary insns.
	(function_info::create_insn): New.
	* rtl-ssa/changes.h (class insn_change): Make function_info a
	friend class.
	* rtl-ssa/functions.h (function_info): Declare new entry points:
	create_set, create_insn.  Declare new change_alloc helper.
	* rtl-ssa/insns.cc (insn_info::print_full): Identify temporary insns in
	dump.
	* rtl-ssa/insns.h (insn_info): Add new m_is_temp flag and accompanying
	is_temporary accessor.
	* rtl-ssa/internals.inl (insn_info::insn_info): Initialize m_is_temp to
	false.
	* rtl-ssa/member-fns.inl (function_info::change_alloc): New.
	* rtl-ssa/movement.h (restrict_movement_for_defs_ignoring): Add
	handling for temporary defs.

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