Tweak reachability to only warn after the first reachable case

In the event that the first case is unreachable, the logic descended to
`isSubspace(covered, prevs)` where both are the Empty Space, emitting
warnings.  Instead we implement reachability like the Scala 2 compiler
defines it:

    // a case is unreachable if it implies its preceding cases

Author: dwijnand

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -922,10 +922,13 @@ class SpaceEngine(using Context) extends SpaceLogic {
 
     (1 until cases.length).foreach { i =>
       val pat = cases(i).pat
-
-      if (pat != EmptyTree) { // rethrow case of catch uses EmptyTree
-        val prevs = Or(spaces.take(i))
-        val curr = project(pat)
+      val prevs = Or(spaces.take(i))
+      if (pat != EmptyTree // rethrow case of catch uses EmptyTree
+        && simplify(intersect(prevs, targetSpace)) != Empty
+        // it's required that at one of the previous cases is reachable (its intersected Space isn't Empty)
+        // because if all the previous cases are unreachable then case i can't be unreachable
+      ) {
+        val curr = project(pat) // TODO(dnw) reuse `spaces(i)` & avoid re-computing? Or is mutability present?
 
         debug.println(s"---------------reachable? ${show(curr)}")
         debug.println(s"prev: ${show(prevs)}")