tree-optimization/119589 - alignment analysis for VF > 1 and VMAT_STRIDED_SLP

tree-optimization/119589 - alignment analysis for VF > 1 and VMAT_STRIDED_SLP

The following fixes the alignment analysis done by the VMAT_STRIDED_SLP
code which for the case of VF > 1 currently relies on dataref analysis
which assumes consecutive accesses.  But the code generation advances
by DR_STEP between each iteration which requires us to assess that
individual DR_STEP preserve the alignment rather than only VF * DR_STEP.
This allows us to use vector aligned accesses in some cases.

	PR tree-optimization/119589
	PR tree-optimization/119586
	PR tree-optimization/119155
	* tree-vect-stmts.cc (vectorizable_store): Verify
	DR_STEP_ALIGNMENT preserves DR_TARGET_ALIGNMENT when
	VF > 1 and VMAT_STRIDED_SLP.  Use vector aligned accesses when
	we can.
	(vectorizable_load): Likewise.