Implement Optional instruction flags

[Nordwald]

Implement Optional instruction flags instead of home-brewn REIL instructions.

For example, the implemented RET instruction could be substituted by an JCC instruction with the optional IOPT_RET flag implemented in openREIL.

I acknowledge the work done on this REIL implementation, but this could increase the interoperability of the two projects and may help to make REIL a more popular IR.

[cnheitman]

Hi! Thanks for your suggestion. It is a feature I've been planning to implement for a while. Indeed, it could increase interoperability, and I think it could ease the development of other features that deal with REIL instructions directly.

Regarding the RET instruction you mentioned, I just want to say it is no longer used in the translations and it will be removed soon.