tree-optimization/111131 - SLP for non-IFN gathers

The following implements SLP vectorization support for gathers
without relying on IFNs being pattern detected (and supported by
the target).  That includes support for emulated gathers but also
the legacy x86 builtin path.

	PR tree-optimization/111131
	* tree-vect-loop.cc (update_epilogue_loop_vinfo): Make
	sure to update all gather/scatter stmt DRs, not only those
	that eventually got VMAT_GATHER_SCATTER set.
	* tree-vect-slp.cc (_slp_oprnd_info::first_gs_info): Add.
	(vect_get_and_check_slp_defs): Handle gathers/scatters,
	adding the offset as SLP operand and comparing base and scale.
	(vect_build_slp_tree_1): Handle gathers.
	(vect_build_slp_tree_2): Likewise.

	* gcc.dg/vect/vect-gather-1.c: Now expected to vectorize
	everywhere.
	* gcc.dg/vect/vect-gather-2.c: Expected to not SLP anywhere.
	Massage the scale case to more reliably produce a different
	one.  Scan for the specific messages.
	* gcc.dg/vect/vect-gather-3.c: Masked gather is also supported
	for AVX2, but not emulated.
	* gcc.dg/vect/vect-gather-4.c: Expected to not SLP anywhere.
	Massage to more properly ensure this.
	* gcc.dg/vect/tsvc/vect-tsvc-s353.c: Expect to vectorize
	everywhere.