vect: Add helper macros for gather/scatter.

vect: Add helper macros for gather/scatter.

This encapsulates the IFN and the builtin-function way of handling
gather/scatter via three defines:

  GATHER_SCATTER_IFN_P
  GATHER_SCATTER_LEGACY_P
  GATHER_SCATTER_EMULATED_P

and introduces a helper define for SLP operand handling as well.

gcc/ChangeLog:

	* tree-vect-slp.cc (GATHER_SCATTER_OFFSET): New define.
	(vect_get_and_check_slp_defs): Use.
	* tree-vectorizer.h (GATHER_SCATTER_LEGACY_P): New define.
	(GATHER_SCATTER_IFN_P): Ditto.
	(GATHER_SCATTER_EMULATED_P): Ditto.
	* tree-vect-stmts.cc (vectorizable_store): Use.
	(vectorizable_load): Use.