Stop transitioning to UNDEF when undefining an instance variable

Cases like this:

```ruby
obj = Object.new
loop do
  obj.instance_variable_set(:@foo, 1)
  obj.remove_instance_variable(:@foo)
end
```

can cause us to use many more shapes than we want (and even run out).
This commit changes the code such that when an instance variable is
removed, we'll walk up the shape tree, find the shape, then rebuild any
child nodes that happened to be below the "targetted for removal" IV.

This also requires moving any instance variables so that indexes derived
from the shape tree will work correctly.

Co-Authored-By: Jemma Issroff <jemmaissroff@gmail.com>
Co-authored-by: John Hawthorn <jhawthorn@github.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 86d3c991f4..ec90e67373 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -1416,7 +1416,7 @@ vm_setivar(VALUE obj, ID id, VALUE val, shape_id_t dest_shape_id, attr_index_t i
                 rb_shape_t *dest_shape = rb_shape_get_shape_by_id(dest_shape_id);
                 shape_id_t source_shape_id = dest_shape->parent_id;
 
-                RUBY_ASSERT(dest_shape->type == SHAPE_IVAR || dest_shape->type == SHAPE_IVAR_UNDEF);
+                RUBY_ASSERT(dest_shape->type == SHAPE_IVAR);
 
                 if (shape_id == source_shape_id && dest_shape->edge_name == id) {
                     RUBY_ASSERT(dest_shape_id != INVALID_SHAPE_ID && shape_id != INVALID_SHAPE_ID);