Emit unreachable tuple.make properly (#2701)

We previously thought unreachable `tuple.make` instructions did not
require special unreachable handling, but consider the following wast:

```
(module
 (func $foo
  (tuple.make
   (unreachable)
   (i32.const 42)
  )
 )
)
```

This validates because the only expression in the body is unreachable,
but when it is emitted as a binary it becomes

```
unreachable
i32.const 42
```

This does not validate because it ends with an i32, but the function
expected an empty stack at the end. The fix is to emit an extra
`unreachable` after unreachable `tuple.make`
instructions. Unfortunately it is impossible to write a test for this
right now because the binary parser silently drops the `i32.const 42`,
making the function valid again.

Author: tlively

src/wasm-stack.h

@@ -809,8 +809,10 @@ void BinaryenIRWriter<SubType>::visitTupleMake(TupleMake* curr) {
   for (auto* operand : curr->operands) {
     visit(operand);
   }
-  // No need to handle unreachable since we don't actually emit an instruction
   emit(curr);
+  if (curr->type == Type::unreachable) {
+    emitUnreachable();
+  }
 }
 
 template<typename SubType>