I see that in WAVM/WAVM#187 they have switched the assert_return_canonical_nan and assert_return_arithmetic_nan to support a type parameter for v128s. It looks like two different styles are discussed there:

• (assert_return_canonical_nan (invoke ...) f32x4)
• (assert_return_canonical_nan_f32x4 (invoke ...))

We should pick one of these and add it to ast/wast.rs; it looks like the comments in WAVM/WAVM#187 lean towards the former but I do see that @Honry used the latter in https://github.com/WebAssembly/simd/blob/master/test/core/simd/simd_f32x4.wast#L994 and following.