Improve code generation of strided SLP loads

Improve code generation of strided SLP loads

This avoids falling back to elementwise accesses for strided SLP
loads when the group size is not a multiple of the vector element
size.  Instead we can use a smaller vector or integer type for the load.

For stores we can do the same though restrictions on stores we handle
and the fact that store-merging covers up makes this mostly effective
for cost modeling which shows for gcc.target/i386/vect-strided-3.c
which we now vectorize with V4SI vectors rather than just V2SI ones.

For all of this there's still the opportunity to use non-uniform
accesses, say for a 6-element group with a VF of two do
V4SI, { V2SI, V2SI }, V4SI.  But that's for a possible followup.

	* tree-vect-stmts.cc (get_group_load_store_type): Consistently
	use VMAT_STRIDED_SLP for strided SLP accesses and not
	VMAT_ELEMENTWISE.
	(vectorizable_store): Adjust VMAT_STRIDED_SLP handling to
	allow not only half-size but also smaller accesses.
	(vectorizable_load): Likewise.

	* gcc.target/i386/vect-strided-1.c: New testcase.
	* gcc.target/i386/vect-strided-2.c: Likewise.
	* gcc.target/i386/vect-strided-3.c: Likewise.
	* gcc.target/i386/vect-strided-4.c: Likewise.