externally_visible_p wasn't the correct predicate to use (even if it
worked), instead we should use DECL_EXTERNAL || TREE_PUBLIC.

2020-05-07  Richard Biener  <rguenther@suse.de>

	PR ipa/94947
	* tree-ssa-structalias.c (refered_from_nonlocal_fn): Use
	DECL_EXTERNAL || TREE_PUBLIC instead of externally_visible.
	(refered_from_nonlocal_var): Likewise.
	(ipa_pta_execute): Likewise.