Scala.js: Fix a codegen bug for switch matches in statement position.

The codegen chooses `jstpe.NoType` (`void`) over the declared type
of a `Match` node. That is normally fine, except if the declared
type was `Nothing`, in which case we must retain it, because the
enclosing statement might be demanding an expression of type
`Nothing`.

Author: sjrd

compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -2277,9 +2277,11 @@ class JSCodeGen()(using genCtx: Context) {
         abortMatch(s"Invalid selector type ${genSelector.tpe}")
     }
 
-    val resultType =
-      if (isStat) jstpe.NoType
-      else toIRType(tree.tpe)
+    val resultType = toIRType(tree.tpe) match {
+      case jstpe.NothingType => jstpe.NothingType // must take priority over NoType below
+      case _ if isStat       => jstpe.NoType
+      case resType           => resType
+    }
 
     var clauses: List[(List[js.Tree], js.Tree)] = Nil
     var optDefaultClause: Option[js.Tree] = None