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, 0 insertions, 1 deletions

diff --git a/tool/mjit/bindgen.rb b/tool/mjit/bindgen.rb
index b846a91664..c093cd423c 100755
--- a/tool/mjit/bindgen.rb
+++ b/tool/mjit/bindgen.rb
@@ -359,7 +359,6 @@ generator = BindingGenerator.new(
       SHAPE_FROZEN
       SHAPE_INITIAL_CAPACITY
       SHAPE_IVAR
-      SHAPE_IVAR_UNDEF
       SHAPE_ROOT
     ],
     ULONG: %w[