Fix crash when evacuating generic ivar

When transitioning generic instance variable objects to too complex, we
set the shape first before performing inserting the new gen_ivtbl. The
st_insert for the new gen_ivtbl could allocate and cause a GC. If that
happens, then it will crash because the object will have a too complex
shape but not yet be backed by a st_table.

This commit changes the order so that the insert happens first before
the new shape is set.

The following script reproduces the issue:

```
o = []
o.instance_variable_set(:@a, 1)

i = 0
o = Object.new
while RubyVM::Shape.shapes_available > 0
  o.instance_variable_set(:"@i#{i}", 1)
  i += 1
end

ary = 1_000.times.map { [] }

GC.stress = true
ary.each do |o|
  o.instance_variable_set(:@a, 1)
  o.instance_variable_set(:@b, 1)
end
```

test/ruby/test_shapes.rb

@@ -277,6 +277,28 @@ class TestShapes < Test::Unit::TestCase
     end;
   end
 
+  def test_gc_stress_during_evacuate_generic_ivar
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      [].instance_variable_set(:@a, 1)
+
+      i = 0
+      o = Object.new
+      while RubyVM::Shape.shapes_available > 0
+        o.instance_variable_set(:"@i#{i}", 1)
+        i += 1
+      end
+
+      ary = 10.times.map { [] }
+
+      GC.stress = true
+      ary.each do |o|
+        o.instance_variable_set(:@a, 1)
+        o.instance_variable_set(:@b, 1)
+      end
+    end;
+  end
+
   def test_run_out_of_shape_for_module_ivar
     assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
     begin;

variable.c

@@ -1399,12 +1399,12 @@ rb_obj_convert_to_too_complex(VALUE obj, st_table *table)
 
             struct gen_ivtbl *ivtbl = xmalloc(sizeof(struct gen_ivtbl));
             ivtbl->as.complex.table = table;
+            st_insert(gen_ivs, (st_data_t)obj, (st_data_t)ivtbl);
 #if SHAPE_IN_BASIC_FLAGS
             rb_shape_set_shape_id(obj, OBJ_TOO_COMPLEX_SHAPE_ID);
 #else
             ivtbl->shape_id = OBJ_TOO_COMPLEX_SHAPE_ID;
 #endif
-            st_insert(gen_ivs, (st_data_t)obj, (st_data_t)ivtbl);
         }
         RB_VM_LOCK_LEAVE();
     }