tree-optimization/103544 - SLP reduction chain as SLP reduction issue

tree-optimization/103544 - SLP reduction chain as SLP reduction issue

When SLP reduction chain vectorization support added handling of
an outer conversion in the chain picking a failed reduction up
as SLP reduction that broke the invariant that the whole reduction
was forward reachable.  The following plugs that hole noting
a future enhancement possibility.

2021-12-06  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103544
	* tree-vect-slp.c (vect_analyze_slp): Only add a SLP reduction
	opportunity if the stmt in question is the reduction root.
	(dot_slp_tree): Add missing check for NULL child.

	* gcc.dg/vect/pr103544.c: New testcase.