Trailing `if` on unit `switch` is ignored

[bbannier]

We allow code with a unit switch with a trailing if, e.g.,

public type X = unit {
    switch {
        -> a: b"A";
        -> b: b"B";
        -> c: b"C";
    } if(False) ;
};

The condition is however silently ignored; for above False I would expect X to consume no data but it does

$ printf 'A' | spicy-dump -d foo.spicy
foo::X {
  a: A
}

We should either interpret such conditions correctly or reject them outright.