IPA PTA stats enhancement and non-details dump slimming

The following keeps track of the number of edges we avoid to create
because they redundandly feed ESCAPED.  It also avoids printing
a header for -details when not using -details.

	* tree-ssa-structalias.cc (constraint_stats::num_avoided_edges):
	New.
	(add_graph_edge): Count redundant edges we avoid to create.
	(dump_sa_stats): Dump them.
	(ipa_pta_execute): Do not dump generating constraints when
	we are not dumping them.