Yes I think this would be useful for solving the "order of optimization passes" problem. Right now I just treat my compiler as a black box. But if the guts were exposed a little bit, the compiler and I could work together to optimize a piece of code.

I could give it a few hints about what order might work, and it could show me what the intermediate representations are so that I can keep them in mind while writing code. There are patterns in C/C++ that trip up compilers, like unintentional aliasing or out params, and I could try to avoid them to help the optimizer. On the other hand, if the optimizer doesn't are, I will use them for my own convenience.

I suppose Clang has a pretty well-defined interface between passes, in the LLVM IR, so you can do this to some degree now. But it is a little obscure and probably version-dependent.