fix alternatives

Author: sjrd

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -785,7 +785,7 @@ object PatternMatcher {
       object AlternativesPlan {
         def unapply(plan: LabeledPlan): Option[(List[Tree], Plan)] = {
           plan.expr match {
-            case SeqPlan(LabeledPlan(innerLabel, innerPlan), ons) =>
+            case SeqPlan(LabeledPlan(innerLabel, innerPlan), ons) if !canFallThrough(ons) =>
               val outerLabel = plan.sym
               val alts = List.newBuilder[Tree]
               def rec(innerPlan: Plan): Boolean = innerPlan match {