tree-optimization/111764 - wrong reduction vectorization

The following removes a misguided attempt to allow x + x in a reduction
path, also allowing x * x which isn't valid.  x + x actually never
arrives this way but instead is canonicalized to 2 * x.  This makes
reduction path handling consistent with how we handle the single-stmt
reduction case.

	PR tree-optimization/111764
	* tree-vect-loop.cc (check_reduction_path): Remove the attempt
	to allow x + x via special-casing of assigns.

	* gcc.dg/vect/pr111764.c: New testcase.