Fix regression in switch pattern translation

The performance optimization in scala#6607 assumed that a translated
match would always be a `Match` node itself, but it can also be
a `{ synthetic val x1 = ...; x1 match { .. } }` block.

Author: retronym

src/compiler/scala/tools/nsc/transform/patmat/PatternMatching.scala

@@ -65,7 +65,13 @@ trait PatternMatching extends Transform
           // Keep 2.12 behaviour of using wildcard expected type, recomputing the LUB, then throwing it away for the continuations plugins
           // but for the rest of us pass in top as the expected type to avoid waste.
           val pt = if (origTp <:< definitions.AnyTpe) definitions.AnyTpe else WildcardType
-          localTyper.typed(translated, pt) setType origTp
+          localTyper.typed(translated, pt) match {
+            case b @ Block(stats, m: Match) =>
+              b.setType(origTp)
+              m.setType(origTp)
+              b
+            case tree => tree setType origTp
+          }
         } catch {
           case x: (Types#TypeError) =>
             // TODO: this should never happen; error should've been reported during type checking

test/files/run/patmat-origtp-switch.check

@@ -11,7 +11,7 @@ package <empty>{<empty>.type} {
         case 0{Int(0)} => a{A}
         case 1{Int(1)} => b{A with C}
         case _{Int} => throw new MatchError{MatchError}{(obj: Any)MatchError}(x1{Int}){MatchError}{Nothing}
-      }{Any}
+      }{A}
     }{A}
   }
 }