tree-optimization/114027 - conditional reduction chain

When we classify a conditional reduction chain as CONST_COND_REDUCTION
we fail to verify all involved conditionals have the same constant.
That's a quite unlikely situation so the following simply disables
such classification when there's more than one reduction statement.

	PR tree-optimization/114027
	* tree-vect-loop.cc (vecctorizable_reduction): Use optimized
	condition reduction classification only for single-element
	chains.

	* gcc.dg/vect/pr114027.c: New testcase.