Fix #15107: Avoid re-emitting a LineNumber after only LabelNodes.

There was already some deduplication code to avoid consecutive
`LineNumber` nodes. However, it can happen that `LabelNode`s
appear in-between. In that case, we also want to deduplicate the
`LineNumber`s, since labels do not actually contribute to the
final bytecode.

compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala

@@ -556,11 +556,17 @@ trait BCodeSkelBuilder extends BCodeHelpers {
         case _ => false } )
     }
     def lineNumber(tree: Tree): Unit = {
+      @tailrec
+      def getNonLabelNode(a: asm.tree.AbstractInsnNode): asm.tree.AbstractInsnNode = a match {
+        case a: asm.tree.LabelNode => getNonLabelNode(a.getPrevious)
+        case _                     => a
+      }
+
       if (!emitLines || !tree.span.exists) return;
       val nr = ctx.source.offsetToLine(tree.span.point) + 1
       if (nr != lastEmittedLineNr) {
         lastEmittedLineNr = nr
-        lastInsn match {
+        getNonLabelNode(lastInsn) match {
           case lnn: asm.tree.LineNumberNode =>
             // overwrite previous landmark as no instructions have been emitted for it
             lnn.line = nr

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -1622,7 +1622,6 @@ class DottyBytecodeTests extends DottyBytecodeTest {
       val instructions = instructionsFromMethod(method).filter(_.isInstanceOf[LineNumber])
 
       val expected = List(
-        LineNumber(2, Label(0)),
         LineNumber(3, Label(0)),
         LineNumber(4, Label(5)), // case y =>
         LineNumber(5, Label(9)),
@@ -1664,7 +1663,6 @@ class DottyBytecodeTests extends DottyBytecodeTest {
       val instructions = instructionsFromMethod(method).filter(_.isInstanceOf[LineNumber])
 
       val expected = List(
-        LineNumber(2, Label(0)),
         LineNumber(3, Label(0)),
         LineNumber(4, Label(5)), // case a if a == 3 =>
         LineNumber(5, Label(15)),