Matching strings should emit switches (as in Scala 2)

[harpocrates]

This was initially reported in scala/bug#11740, then fixed in scala/scala#8451, but never ported to Dotty (perhaps because the changes collide a little without the new pattern matching).

I would expect the following to compile into a lookupswitch based on the hashCode of the cases, as it does in Scala 2 (and as Java, Kotlin, etc. also do).

def testMatch(s: String): Int = (s: @switch) match {
  case "S0" => 123
  case "S1" => 121
  case "S2" => 193
  case "S3" => 223
  case "S4" => 423
  case "S5" => 153
  case "S6" => 143
  case "S7" => 133
}

To further confuse the story, this comment from the JS codgen appears to suggest that at some point the Scala.js backend would've received and emitted a switch over strings (although I can't reproduce that behavior, even on Scala 2 and with @switch supposedly claiming a switch will be emitted).

[harpocrates]

I'm going to take a stab at this, but I'm opening an issue anyways in case I run out of steam (since this really should be addressed).

[nicolasstucki]

@sjrd should we keep those matched as Match trees in the PatternMatcher and then compile them to bytecode in the backend directly. Or, should we transform the match into a match on the hashCode in PatternMatcher and keep the switches on Int for the backend. What is simpler for Scala.js? I would assume the second option.

[sjrd]

We should do as Scala 2 does, i.e., keep those as Match trees in the PatternMatcher, and then compile them to bytecode in the backend directly. Scala.js has its own way of compiling those, which does not use the hashCode.

[bishabosha]

I started with ef06795 but never got to refining it