tree-optimization/103864 - SLP reduction of reductions with conversions

tree-optimization/103864 - SLP reduction of reductions with conversions

This generalizes the fix for PR103544 to also cover reductions that
are not reduction chains and does not consider reductions wrapped in
sign conversions for SLP reduction handling.

2022-01-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103864
	PR tree-optimization/103544
	* tree-vect-slp.c (vect_analyze_slp_instance): Exclude
	reductions wrapped in conversions from SLP handling.
	(vect_analyze_slp): Revert PR103544 change.

	* gcc.dg/vect/pr103864.c: New testcase.