This patch uses the vect_memory_access_type from patch 6 to represent
the effect of a negative contiguous stride or a zero stride.  The latter
is valid only for loads.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.

gcc/
	* tree-vectorizer.h (vect_memory_access_type): Add
	VMAT_INVARIANT, VMAT_CONTIGUOUS_DOWN and VMAT_CONTIGUOUS_REVERSED.
	* tree-vect-stmts.c (compare_step_with_zero): New function.
	(perm_mask_for_reverse): Move further up file.
	(get_group_load_store_type): Stick to VMAT_ELEMENTWISE if the
	step is negative.
	(get_negative_load_store_type): New function.
	(get_load_store_type): Call it.  Add an ncopies argument.
	(vectorizable_mask_load_store): Update call accordingly and
	remove tests for negative steps.
	(vectorizable_store, vectorizable_load): Likewise.  Handle new
	memory_access_types.

From-SVN: r238039