Record gather/scatter scale and base in the SLP tree

Record gather/scatter scale and base in the SLP tree

The main gather/scatter discovery happens at SLP discovery time,
the base address and the offset scale are currently not explicitly
represented in the SLP tree.  This requires re-discovery of them
during vectorizable_store/load.  The following fixes this by
recording this info into the SLP tree.  This allows the main
vect_check_gather_scatter call to be elided from get_load_store_type
and replaced with target support checks for IFN/decl or fallback
emulated mode.

There's vect_check_gather_scatter left in the path using gather/scatter
for strided load/store.  I hope to deal with this later.

	* tree-vectorizer.h (_slp_tree::gs_scale): New.
	(_slp_tree::gs_base): Likewise.
	(SLP_TREE_GS_SCALE): Likewise.
	(SLP_TREE_GS_BASE): Likewise.
	(vect_describe_gather_scatter_call): Declare.
	* tree-vect-slp.cc (_slp_tree::_slp_tree): Initialize
	new members.
	(vect_build_slp_tree_2): Record gather/scatter base and scale.
	(vect_get_and_check_slp_defs): For gather/scatter IFNs
	describe the call to first_gs_info.
	* tree-vect-data-refs.cc (vect_gather_scatter_fn_p): Add
	mode of operation with fixed offset vector type.
	(vect_describe_gather_scatter_call): Export.
	* tree-vect-stmts.cc (get_load_store_type): Do not call
	vect_check_gather_scatter to fill gs_info, instead populate
	from the SLP tree.  Check which of, IFN, decl or fallback
	is supported and record that decision.