Backport Fix #1169: Make IR printer print cases of match tree nicely

It now looks like this:

    match (x1) {
      case 1 | 2 | 3:
        mod:s_Predef$.println__O__V("low");
      case 4:
        mod:s_Predef$.println__O__V("four");
      default:
        mod:s_Predef$.println__O__V("other");
    }

(cherry picked from commit 0d6504b872d191e600ebf638e5fad0fae6db23ce)

Conflicts:

	ir/src/main/scala/scala/scalajs/ir/Printers.scala