I don't think we should ever consider forwarded IC's to be "simple".
Previously, the "simple" flag would be copied to the derived IC and this
happened to cause struct set / get iseqs to write an invalid CC
fastpath:

  https://github.com/tenderlove/ruby/blob/f45eb3dcb9c7d849064cb802953f37e1cf9f3996/vm_insnhelper.c#L4726-L4729

[Bug #20799]