This fixes lack of an escape point of externally declared variables.

2020-05-05  Richard Biener  <rguenther@suse.de>

	PR ipa/94947
	* tree-ssa-structalias.c (ipa_pta_execute): Use
	varpool_node::externally_visible_p ().
	(refered_from_nonlocal_var): Likewise.

	* gcc.dg/torture/pr94947-1.c: New testcase.
	* gcc.dg/torture/pr94947-2.c: Likewise.