Bug 1087549: Give IonScript::refcount_ and associated functions better names. r=jandem

This patch renames:

  - IonScript::refcount_ to invalidationCount_
  - IonScript::refcount to invalidationCount
  - IonScript::incref to incrementInvalidationCount
  - IonScript::decref to decrementInvalidationCount
  - IonScript::offsetOfRefcount to offsetOfInvalidationCount