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.