Handle TYPE_OVERFLOW_UNDEFINED vectorized BB reductions

Handle TYPE_OVERFLOW_UNDEFINED vectorized BB reductions

The following changes the gate to perform vectorization of BB reductions
to use needs_fold_left_reduction_p which in turn requires handling
TYPE_OVERFLOW_UNDEFINED types in the epilogue code generation by
promoting any operations generated there to use unsigned arithmetic.

The following does this, there's currently only v16qi where x86
supports a .REDUC_PLUS reduction for integral modes so I had to
add a x86 specific testcase using GIMPLE IL.

	* tree-vect-slp.cc (vect_slp_check_for_roots): Use
	!needs_fold_left_reduction_p to decide whether we can
	handle the reduction with association.
	(vectorize_slp_instance_root_stmt): For TYPE_OVERFLOW_UNDEFINED
	reductions perform all arithmetic in an unsigned type.

	* gcc.target/i386/vect-reduc-2.c: New testcase.