when we already check ROBJECT_NUMIV(self) is larger than
ROBJECT_EMBED_LEN_MAX at the beginning of the method, because the number
of instance variables for the same object doesn't decrease.

```
$ benchmark-driver -v --rbenv 'before --jit;after --jit' --repeat-count=4 --alternate --output=all benchmark_3000.yml
before --jit: ruby 3.0.0dev (2020-12-23T06:32:19Z master dbb4f199) +JIT [x86_64-linux]
after --jit: ruby 3.0.0dev (2020-12-23T07:45:42Z master 95e866c098) +JIT [x86_64-linux]
last_commit=Skip checking ROBJECT_EMBED
Calculating -------------------------------------
                              before --jit           after --jit
Optcarrot 3000 frames   102.34091772397872    102.77738408379015 fps
                        103.37784821624231    105.46530219076179
                        104.39567016876369    106.43712452152215
                        105.31782092252713    106.54986150067481
```