tree-optimization/105431 - another overflow in powi handling

This avoids undefined signed overflow when calling powi_as_mults_1.

2022-04-29  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105431
	* tree-ssa-math-opts.cc (powi_as_mults_1): Make n unsigned.
	(powi_as_mults): Use absu_hwi.
	(gimple_expand_builtin_powi): Remove now pointless n != -n
	check.