tree-optimization/38474 - improve PTA varinfo sorting

tree-optimization/38474 - improve PTA varinfo sorting

This improves a previous heuristic to sort address-taken variables
first (because those appear in points-to bitmaps) by tracking which
variables appear in ADDRESSOF constraints (there's also
graph->address_taken but that's computed only later).

This shaves off 30s worth of compile-time for the full testcase in
PR38474 (which then still takes 965s to compile at -O2).

2021-02-16  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/38474
	* tree-ssa-structalias.c (variable_info::address_taken): New.
	(new_var_info): Initialize address_taken.
	(process_constraint): Set address_taken.
	(solve_constraints): Use the new address_taken flag rather
	than is_reg_var for sorting variables.
	(dump_constraint): Dump the variable number if the name
	is just NULL.