Better support for guards in switches

#4982 Added some support for guards in switches. However this still doesn't work:
```scala
import scala.annotation.switch

class Test {
  def succ_guard(c: Int) = (c: @switch) match { // warning: Could not emit switch for @switch annotated match
    case 1 | 2 | 3   => true
    case x if x == 4 => true
    case _           => false
  }
}
```