This helps hit inline method caches more frequently.  Before this
commit:

```
[aaron@TC ruby (trunk)]$ time ./ruby -v benchmark/bm_vm2_poly_singleton.rb
ruby 2.4.0dev (2016-09-12 trunk 56141) [x86_64-darwin15]

real  0m3.679s
user  0m3.632s
sys 0m0.022s
```

After this commit:

```
[aaron@TC ruby (trunk)]$ time ./ruby -v benchmark/bm_vm2_poly_singleton.rb
ruby 2.4.0dev (2016-09-12 trunk 56141) [x86_64-darwin15]
last_commit=Copy the serial number from the super class to the singleton class

real  0m2.246s
user  0m2.203s
sys 0m0.020s
```

[Feature #12364]
[ruby-core:75425]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56144 b2dd03c8-39d4-4d8f-98ff-823fe69b080e