> So is GCC adding a toolchain similar to the one LLVM has around its bitcode?

Not as far as I can tell.

> In this instance, a way to write a text representation of an internal data structure that represents code, run a single optimization pass over that data structure, dump the result, compare the result with serialized text representation of "expected value"?

That's already existed for ages. See the -fdump-tree-* family of functions. It also doesn't do anything llvm-like.