[4/7] Add a gather_scatter_info structure

[4/7] Add a gather_scatter_info structure

This patch just refactors the gather/scatter support so that all
information is in a single structure, rather than separate variables.
This reduces the number of arguments to a function added in patch 6.

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

gcc/
	* tree-vectorizer.h (gather_scatter_info): New structure.
	(vect_check_gather_scatter): Return a bool rather than a decl.
	Replace return-by-pointer arguments with a single
	gather_scatter_info *.
	* tree-vect-data-refs.c (vect_check_gather_scatter): Likewise.
	(vect_analyze_data_refs): Update call accordingly.
	* tree-vect-stmts.c (vect_mark_stmts_to_be_vectorized): Likewise.
	(vectorizable_mask_load_store): Likewise.  Also record the
	offset dt and vectype in the gather_scatter_info.
	(vectorizable_store): Likewise.
	(vectorizable_load): Likewise.

From-SVN: r238036