[asm-cherry-pick] Ensure instructions belong only to one list

Fail early when adding an instruction that already belongs to a
different instruction list.

Originally added in
lrytz/scala@eccd0dc

Author: lrytz

src/main/java/scala/tools/asm/tree/InsnList.java

@@ -223,6 +223,14 @@ public void set(final AbstractInsnNode oldInsnNode, final AbstractInsnNode newIn
    * @param insnNode an instruction, <i>which must not belong to any {@link InsnList}</i>.
    */
   public void add(final AbstractInsnNode insnNode) {
+    if(insnNode.previousInsn != null || insnNode.nextInsn != null) {
+      // Adding an instruction that still refers to others (in the same or another InsnList) leads to hard to debug bugs.
+      // Initially everything may look ok (e.g. iteration follows `next` thus a stale `prev` isn't noticed).
+      // However, a stale link brings the doubly-linked into disarray e.g. upon removing an element,
+      // which results in the `next` of a stale `prev` being updated, among other failure scenarios.
+      // Better fail early.
+      throw new RuntimeException("Instruction " + insnNode + " already belongs to some InsnList.");
+    }
     ++size;
     if (lastInsn == null) {
       firstInsn = insnNode;