tree-optimization/97812 - fix range query in VRP assert discovery

This makes sure to properly extend the input range before seeing
whether it fits the target.

2020-11-13  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97812
	* tree-vrp.c (register_edge_assert_for_2): Extend the range
	according to its sign before seeing whether it fits.

	* gcc.dg/torture/pr97812.c: New testcase.