[Wasm GC] Fix struct.set / ref.as_non_null ordering issue

[kripken]

If traps can happen, then we can't always remove a trap on null
on the ref input to struct.set, since it has two children,

(struct.set
  (ref.as_non_null X)
  (call $foo))

Removing the ref.as would not prevent a trap, as the struct.set
will trap, but it does move the trap to after the call which is bad.

[tlively]

Would it be sufficient to check for control flow side effects here? Updating a local certainly won't matter, for instance. I guess updating a global might matter, though?

But I guess you'll say that side effects are rare enough that it doesn't really matter and we should do the simplest thing here 👍

[kripken]

Good point, we could do a little better here. We can also ignore traps, since traps are ok to reorder. I added a TODO as it seems low priority, and would probably require some tinkering in effects.h.