tree-optimization/108950 - widen-sum reduction ICE

When we end up with a widen-sum with an invariant smaller operand
the reduction code uses a wrong vector type for it, causing
IL checking ICEs.  The following fixes that and the inefficiency
of using a widen-sum with a widenend invariant operand as well
by actually performing the check the following comment wants.

	PR tree-optimization/108950
	* tree-vect-patterns.cc (vect_recog_widen_sum_pattern):
	Check oprnd0 is defined in the loop.
	* tree-vect-loop.cc (vectorizable_reduction): Record all
	operands vector types, compute that of invariants and
	properly update their SLP nodes.

	* gcc.dg/vect/pr108950.c: New testcase.