Correctly handle store IFNs in vect_get_vector_types_for_stmt

Currently vect_get_vector_types_for_stmt only special-cases
IFN_MASK_STORE but there are now very many variants and simply
passing analysis without setting *VECTYPE will ICE duing SLP
discovery (noticed with IFN_SCATTER_STORE).  The following
properly uses internal_store_fn_p.  I also noticed we're
unnecessarily handing those again to determine the scalar type
but there should always be a data reference for them.

	* tree-vect-stmts.cc (vect_get_vector_types_for_stmt):
	Handle all internal_store_fn_p the same.  Remove special-casing
	for the scalar_type of IFN_MASK_STORE.