WebAssembly: Add the following test

[Keith Miller]

(module
 (func (export "if-then-both-fallthrough") (param $x i32) (param $y i32) (result i32)
       (block $block i32
         (if i32 (i32.eq (get_local $x) (i32.const 0))
           (then (i32.const 1))
           (else
            (i32.const 2)
            (i32.const 1)
            )
           )
         )
       )
 )

We can't add this yet since the current version of the ML prototype does not support the stack at block exit differing the expected return type.
Updating the ML-prototype version is a pain since we will need to update all the opcode numbers and regenerate all the existing tests.

[JF Bastien]

Is this still something that we need to do?