Errors with simplified Switch cases

[dadhi]

see #438 for the context

Single-Test Switch Without a Default Case

If a switch contains only one TestValue and no default case, we get an error during conditional generation. This seems to occur even when such a structure would otherwise be valid.

// FEC throws System.NullReferenceException: Object reference not set to an instance of an object.

var label = Label( "label" );
var block = Block(
    Switch(
        Constant( 1 ),
        SwitchCase( Goto( label ), Constant( 1 ) ) // This gets emitted as a conditional with a null False block
    ),
    Label( label ),
    Constant( 2 )
);

var lambda = Lambda<Func<int>>( block );
var compiledLambda = lambda.CompileFast();

Switch Without Any Test Cases

If a switch contains zero test cases it throws System.ArgumentOutOfRangeException: Index was out of range.. In this case we optimized our code to avoid this scenario.

// FEC throws System.ArgumentOutOfRangeException: Index was out of range.
// WORKAROUND: Do not use empty cases

var block = Block(
    Switch(
        Constant( 1 ),
        []  // empty cases (no body should do this)
    ),
    Constant( 2 )
);

var lambda = Lambda<Func<int>>( block );
var compiledLambda = lambda.CompileFast();