tree-optimization/97241 - fix ICE in reduction vectorization

tree-optimization/97241 - fix ICE in reduction vectorization

The following moves an ad-hoc attempt at discovering the SLP node
for a stmt to the place where we can find it in lock-step when
we find the stmt itself.

2020-09-29  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97241
	* tree-vect-loop.c (vectorizable_reduction): Move finding
	the SLP node for the reduction stmt to a better place.

	* gcc.dg/vect/pr97241.c: New testcase.