Enable the simd_i32x4_trunc_sat_f64x2 test for AArch64

Also, reorganize the AArch64-specific VCode instructions for unary
narrowing and widening vector operations, so that they are more
straightforward to use.

Copyright (c) 2021, Arm Limited.

cranelift/interpreter/src/step.rs

@@ -530,6 +530,7 @@ where
             arg(0)?,
             ValueConversionKind::Truncate(ctrl_ty),
         )?),
+        Opcode::Uunarrow => unimplemented!("Uunarrow"),
         Opcode::Uextend => assign(Value::convert(
             arg(0)?,
             ValueConversionKind::ZeroExtend(ctrl_ty),