* insns.def: unify ic_constant_serial and ic_class_serial into one field
  ic_serial. This is possible because these fields are only ever used
  exclusively with each other.

* insns.def: ditto
* vm_core.h: ditto
* vm_insnhelper.c: ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43596 b2dd03c8-39d4-4d8f-98ff-823fe69b080e