tree-optimization/117767 - VMAT_STRIDED_SLP and alignment

tree-optimization/117767 - VMAT_STRIDED_SLP and alignment

This plugs another hole in alignment checking with VMAT_STRIDED_SLP.
When using an alternate load or store type we have to check whether
that's supported with respect to required vector alignment.

	PR tree-optimization/117767
	* tree-vect-stmts.cc (vectorizable_store): Check for supported
	alignment before using a an alternate store vector type.
	(vectorizable_load): Likewise for loads.

	* gcc.dg/vect/pr117767.c: New testcase.