Fix missing write barrier in iseq instruction list

There's a missing write barrier for operands in the iseq instruction
list, which can cause crashes.

It can be reproduced when Ruby is compiled with `-DRUBY_DEBUG_ENV=1`.
Using the following command:

```
RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR=0 RUBY_DEBUG=gc_stress ruby -w --disable=gems -Itool/lib -W0 test.rb
```

The following script crashes:

```
require "test/unit"
```

compile.c

@@ -1282,6 +1282,32 @@ new_adjust_body(rb_iseq_t *iseq, LABEL *label, int line)
     return adjust;
 }
 
+static void
+iseq_insn_each_markable_object(INSN *insn, void (*func)(VALUE *, VALUE), VALUE data)
+{
+    const char *types = insn_op_types(insn->insn_id);
+    for (int j = 0; types[j]; j++) {
+        char type = types[j];
+        switch (type) {
+          case TS_CDHASH:
+          case TS_ISEQ:
+          case TS_VALUE:
+          case TS_IC: // constant path array
+          case TS_CALLDATA: // ci is stored.
+            func(&OPERAND_AT(insn, j), data);
+            break;
+          default:
+            break;
+        }
+    }
+}
+
+static void
+iseq_insn_each_object_write_barrier(VALUE *obj_ptr, VALUE iseq)
+{
+    RB_OBJ_WRITTEN(iseq, Qundef, *obj_ptr);
+}
+
 static INSN *
 new_insn_core(rb_iseq_t *iseq, const NODE *line_node,
               int insn_id, int argc, VALUE *argv)
@@ -1299,6 +1325,9 @@ new_insn_core(rb_iseq_t *iseq, const NODE *line_node,
     iobj->operands = argv;
     iobj->operand_size = argc;
     iobj->sc_state = 0;
+
+    iseq_insn_each_markable_object(iobj, iseq_insn_each_object_write_barrier, (VALUE)iseq);
+
     return iobj;
 }
 
@@ -10735,6 +10764,12 @@ iseq_build_kw(rb_iseq_t *iseq, VALUE params, VALUE keywords)
     return keyword;
 }
 
+static void
+iseq_insn_each_object_mark_and_move(VALUE *obj_ptr, VALUE _)
+{
+    rb_gc_mark_and_move(obj_ptr);
+}
+
 void
 rb_iseq_mark_and_move_insn_storage(struct iseq_compile_data_storage *storage)
 {
@@ -10761,23 +10796,7 @@ rb_iseq_mark_and_move_insn_storage(struct iseq_compile_data_storage *storage)
             iobj = (INSN *)&storage->buff[pos];
 
             if (iobj->operands) {
-                int j;
-                const char *types = insn_op_types(iobj->insn_id);
-
-                for (j = 0; types[j]; j++) {
-                    char type = types[j];
-                    switch (type) {
-                      case TS_CDHASH:
-                      case TS_ISEQ:
-                      case TS_VALUE:
-                      case TS_IC: // constant path array
-                      case TS_CALLDATA: // ci is stored.
-                        rb_gc_mark_and_move(&OPERAND_AT(iobj, j));
-                        break;
-                      default:
-                        break;
-                    }
-                }
+                iseq_insn_each_markable_object(iobj, iseq_insn_each_object_mark_and_move, (VALUE)0);
             }
             pos += (int)size;
         }