Move negative stride bias out of dr_misalignment

Move negative stride bias out of dr_misalignment

This moves applying of a bias for negative stride accesses out of
dr_misalignment in favor of a more general optional offset argument.
The negative bias is now computed by get_load_store_type and applied
accordingly to determine the alignment support scheme.  Likewise
the peeling/versioning code is adjusted albeit that still assumes
we'll end up with VMAT_CONTIGUOUS_DOWN or VMAT_CONTIGUOUS_REVERSE
but at least when not so (VMAT_STRIDED_SLP is one possibility) then
get_load_store_type will _not_ falsely report an aligned access but
instead an access with known misalignment.

This fixes PR96109.

2021-10-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96109
	* tree-vectorizer.h (dr_misalignment): Add optional offset
	parameter.
	* tree-vect-data-refs.c (dr_misalignment): Likewise.  Remove
	offset applied for negative stride accesses.
	(vect_enhance_data_refs_alignment): Compute negative stride
	access offset and pass it to dr_misalignment.
	* tree-vect-stmts.c (get_negative_load_store_type): Pass
	negative offset to dr_misalignment.
	(get_group_load_store_type): Likewise.
	(get_load_store_type): Likewise.
	(vectorizable_store): Remove asserts about alignment.
	(vectorizable_load): Likewise.