[IR] Specialized yieldSpecific code for common cases

* convertValueToArgArgArray is the generic code that converts
  yield arg(s) (as is or that has been recast into a RubyArray)
  into IRubyObject[] args array. This patch eliminated some
  unnecessary construction of a RubyArray object on some code paths

* Discovered this when I was looking at whether any of that code
  could be wrapped in IR instructions.