Some refactoring and definitions to use for (unknown) DR_MISALIGNMENT,

gcc/ChangeLog:

2017-04-26  Robin Dapp  <rdapp@linux.vnet.ibm.com>

	* tree-data-ref.h (struct data_reference): Create DR_HAS_NEGATIVE_STEP.
	* tree-vectorizer.h (dr_misalignment): Define DR_MISALIGNMENT.
	* tree-vect-data-refs.c (vect_compute_data_ref_alignment): Use.
	(vect_update_misalignment_for_peel): Use.
	(vect_enhance_data_refs_alignment): Use.
	(vect_no_alias_p): Use.
	(vect_duplicate_ssa_name_ptr_info): Use.
	(known_alignment_for_access_p): Use.