Remove vconcat and vsplit clif instructions (#5465)

Fixes #5463.

* remove vsplit instruction

* remove vconcat instruction

* remove unsused half/double vector helper functions

* remove unused operand constraints

* delete + inline Type::half_vector method

cranelift/interpreter/src/step.rs

@@ -985,16 +985,6 @@ where
             }
             assign(Value::int(result, ctrl_ty)?)
         }
-        Opcode::Vsplit => {
-            let new_type = ctrl_ty.half_vector().unwrap();
-            let vector = extractlanes(&arg(0)?, ctrl_ty)?;
-            let (high, low) = vector.split_at((ctrl_ty.lane_count() / 2) as usize);
-            assign_multiple(&[
-                vectorizelanes(high, new_type)?,
-                vectorizelanes(low, new_type)?,
-            ])
-        }
-        Opcode::Vconcat => unimplemented!("Vconcat"),
         Opcode::Vselect => assign(vselect(&arg(0)?, &arg(1)?, &arg(2)?, ctrl_ty)?),
         Opcode::VanyTrue => {
             let lane_ty = ctrl_ty.lane_type();