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.

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