tree-optimization/122406 - incomplete handling of reduction chain with conversion

tree-optimization/122406 - incomplete handling of reduction chain with conversion

The following fixes the mixup between reduction operation and conversion
wrapping a reduction chain.  This also exposes a missed optimization
but I'm going to fix that in a followup.

	PR tree-optimization/122406
	* tree-vect-slp.cc (vect_analyze_slp_reduc_chain): Create
	the SLP nodes for the conversions around the reduction
	operation if required.

	* gcc.dg/vect/vect-pr122406-1.c: New testcase.
	* gcc.dg/vect/vect-pr122406-2.c: Likewise.