boxing/unboxing vectors

  let match_composite ty = match classify_type ty with
      TypeKind.Vector ->
      let size = vector_size (type_of llval) in
      let ptr = build_in_bounds_gep value_ptr (idx 3) "boxptr" builder in
      let vec_ptr = build_in_bounds_gep ptr [| const_int i32_type 0 |]
                                        "vecptr" builder in
      let new_vec = build_alloca (vector_type i64_type size) "vec" builder in
      let new_vec_ptr = build_in_bounds_gep new_vec (idx 0) "vecptr" builder in
      let rhvector_type size = pointer_type (vector_type i64_type size) in
      ignore (build_store new_vec_ptr vec_ptr builder);
      let el = build_load ptr "el" builder in
      build_bitcast el (rhvector_type size) "dst" builder

let unbox_vec llval =
  let idx0 = [| const_int i32_type 0; const_int i32_type 3 |] in
  let ptr = build_in_bounds_gep llval idx0 "boxptr" builder in
  let el = build_load ptr "el" builder in
  let rhvector_type size = pointer_type (vector_type i64_type size) in
  let vec = build_bitcast el (rhvector_type 3) "vecptr" builder in
  build_load vec "load" builder