Better support for guards in switches

[allanrenucci]

#4982 Added some support for guards in switches. However this still doesn't work:

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
  }
}

[allanrenucci]

It actually gets translated to a switch. In this specific case, the warning is spurious. The condition to emit the warning is wrong:
https://github.com/lampepfl/dotty/blob/e4735d2f758ac5701484be7bc068ee32defac74f/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala#L917

[sjrd]

Try adding more cases without guard after the one with a guard.

[sjrd]

Dumping the document I created with @allanrenucci earlier today: https://gist.github.com/sjrd/16e9cb6c7d7b9a2093cf1521f93f2209