re PR tree-optimization/31605 (VRP eliminates a useful test due with conversion from unsigned int to int)

./:	PR tree-optimization/31605
	* tree-vrp.c (set_value_range): Check that min and max are not
	both overflow infinities.
	(set_value_range_to_value): New static function.
	(extract_range_from_binary_expr): Call set_value_range_to_value.
	(extract_range_from_cond_expr): Likewise.
	(extract_range_from_expr): Likewise.
	(extract_range_from_unary_expr): Likewise.  Don't create a range
	which overflows on both sides.
	(vrp_meet): Check for a useless range.
	(vrp_visit_phi_node): If we see a constant which looks like an
	overflow infinity, turn off the TREE_OVERFLOW flag.
testsuite/:
	PR tree-optimizatoin/31605
	* gcc.c-torture/execute/pr31605.c: New test.

From-SVN: r124128