SI-7433 Fix spurious warning about catching control throwable

In the same vein as SI-6994, such warnings should only be run
in the typer phase, and not in later typechecking (usually in
erasure.)

Why? After translation of pattern matching, we end up with:

    case (ex8 @ _) => {
    <synthetic> val x5: Throwable = ex8;
    case11(){
      if ({
        case14(){
          if (x5.$isInstanceOf[NullPointerException]())
            matchEnd13(true)
          else
            case15()
        };
        case15(){
          if (x5.$isInstanceOf[RuntimeException]())
            matchEnd13(true)
          else
            case16()
        };
        case16(){
          matchEnd13(false)
        };
        matchEnd13(x: Boolean){
          x
        }
      })