re PR tree-optimization/31522 (False overflow warning with phi nodes)

re PR tree-optimization/31522 (False overflow warning with phi nodes)

./:	PR tree-optimization/31522
	* tree-vrp.c (vr_phi_edge_counts): New static variable.
	(vrp_initialize): Allocate vr_phi_edge_counts.
	(vrp_visit_phi_node): Don't push to infinity if we saw a new
	executable edge.  Drop test for all constants.
	(vrp_finalize): Free vrp_phi_edge_counts.
testsuite/:
	PR tree-optimization/31522
	* gcc.dg/Wstrict-overflow-16.c: New test.

From-SVN: r123908