In the D compiler, I realized that while loops could be rewritten as for loops, and so implemented that. The for loops are then rewritten using goto's. This makes the IR a list of expression trees connected by goto's. This data structure makes Data Flow Analysis fairly simple.

An early function inliner I implemented by inlining the IR. When I wrote the D front end, I attempted to do this in the front end. This turned out to be a significantly more complicated problem, and in the end not worth it.

The difficulty with the IR versions is, for error messages, it is impractical to try and issue error messages in the context of the original parse trees. I.e. it's the ancient "turn the hamburger into a cow" problem.