cgraph: Move next/previous from symtab to toplevel_node [PR122955]

cgraph: Move next/previous from symtab to toplevel_node [PR122955]

Currently the GC marker functions don't support chain_next on non-toplevel
tag structures (and does not error out either). So since r16-4747-g529c25ed6e0a06
if there are a lot of chained symtab_nodes (happens most likely with LTO), the GC
marker function could cause a stack overflow because of the recusive nature of the marker.

This fixes the problem by moving next/previous to toplevel_node. I had originally
thought about doing chain_next/chain_prev and using is_a<symtab_node *> to get if
it was symtab_node and then used the next/previous from there. But it was noticed that
asm_node had a next too (though not using chain_next) so adding a previous is not going
to much more space anyways; there will not be many toplevel inline-asm anyways.

Bootstraped and tested on x86_64-linux-gnu.

	PR ipa/122955
gcc/ChangeLog:

	* cgraph.h (toplevel_node): Add next and previous fields.
	Add chain_next and chain_prev to GTY.
	(symtab_node): Remove next and previous field. Remove chain_next and chain_prev
	from the GTY.
	(asm_node): Remove next field.
	(symtab_node::next_defined_symbol): Use save_as_a<symtab_node*> around next.
	(symbol_table::unregister): Likewise
	(FOR_EACH_SYMBOL): Likewise
	(symbol_table::first_defined_symbol): Likewise
	(symbol_table::first_variable): Likewise
	(symbol_table::next_variable): Likewise
	(symbol_table::first_static_initializer): Likewise
	(symbol_table::next_static_initializer): Likewise
	(symbol_table::first_defined_variable): Likewise
	(symbol_table::next_defined_variable): Likewise
	(symbol_table::first_defined_function): Likewise
	(symbol_table::next_defined_function): Likewise
	(symbol_table::first_function): Likewise
	(symbol_table::next_function): Likewise
	(symbol_table::first_function_with_gimple_body): Likewise
	(symbol_table::next_function_with_gimple_body): Likewise
	* cgraphunit.cc (analyze_functions): Likewise
	(output_in_order): Likewise
	* lto-streamer-out.cc (lto_output): Use save_as_a<asm_node*> around next.
	* symtab.cc (symtab_node::verify_symtab_nodes): Likewise.

gcc/lto/ChangeLog:

	* lto-partition.cc (lto_1_to_1_map): Use save_as_a<asm_node*> around next.
	(create_asm_partition): Likewise.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>