Add emulated scatter capability to the vectorizer

Add emulated scatter capability to the vectorizer

This adds a scatter vectorization capability to the vectorizer
without target support by decomposing the offset and data vectors
and then performing scalar stores in the order of vector lanes.
This is aimed at cases where vectorizing the rest of the loop
offsets the cost of vectorizing the scatter.

The offset load is still vectorized and costed as such, but like
with emulated gather those will be turned back to scalar loads
by forwrpop.

	* tree-vect-data-refs.cc (vect_analyze_data_refs): Always
	consider scatters.
	* tree-vect-stmts.cc (vect_model_store_cost): Pass in the
	gather-scatter info and cost emulated scatters accordingly.
	(get_load_store_type): Support emulated scatters.
	(vectorizable_store): Likewise.  Emulate them by extracting
	scalar offsets and data, doing scalar stores.

	* gcc.dg/vect/pr25413a.c: Un-XFAIL everywhere.
	* gcc.dg/vect/vect-71.c: Likewise.
	* gcc.dg/vect/tsvc/vect-tsvc-s4113.c: Likewise.
	* gcc.dg/vect/tsvc/vect-tsvc-s491.c: Likewise.
	* gcc.dg/vect/tsvc/vect-tsvc-vas.c: Likewise.