tree-optimization/98117 - fix range set by vectorization on niter IVs

tree-optimization/98117 - fix range set by vectorization on niter IVs

This avoids the degenerate case of a TYPE_MAX_VALUE latch iteration
count value causing wrong range info for the vector IV.  There's
still the case of VF == 1 where if we don't know whether we hit the
above case we cannot emit a range.

2020-12-07  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98117
	* tree-vect-loop-manip.c (vect_gen_vector_loop_niters):
	Properly handle degenerate niter when setting the vector
	loop IV range.

	* gcc.dg/torture/pr98117.c: New testcase.