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, 0 deletions

diff --git a/common.mk b/common.mk
index 0d7f39e4f0..3c89be7f3f 100644
--- a/common.mk
+++ b/common.mk
@@ -14219,6 +14219,7 @@ shape.$(OBJEXT): {$(VPATH)}st.h
 shape.$(OBJEXT): {$(VPATH)}subst.h
 shape.$(OBJEXT): {$(VPATH)}thread_$(THREAD_MODEL).h
 shape.$(OBJEXT): {$(VPATH)}thread_native.h
+shape.$(OBJEXT): {$(VPATH)}variable.h
 shape.$(OBJEXT): {$(VPATH)}vm_core.h
 shape.$(OBJEXT): {$(VPATH)}vm_debug.h
 shape.$(OBJEXT): {$(VPATH)}vm_opts.h