address review: add comment

Author: liufengyun

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

@@ -77,6 +77,19 @@ class LabelDefs extends MiniPhase {
               assert(t.symbol is Label)
               EmptyTree
             case t: If =>
+              // Trees generated by pattern matcher usually looks like the
+              // following:
+              //
+              //      ...
+              //               }
+              //              else case58(x704)
+              //           }
+              //          else case58(x704)
+              //       }
+              //      else case58(x704)
+              //
+              // First transform on the else branch will make labeldef
+              // inserted on a shallow node.
               val elsep2 = transform(t.elsep)
               val thenp2 = transform(t.thenp)
               cpy.If(tree)(transform(t.cond), thenp2, elsep2)