Another for translation validation:
What about the backends and Clang and Rust frontends?
Work in progress for correctness that's being hit from many, different directions. I don't know of anyone doing the front-ends, though. Not for LLVM, anyway.
So, it's kind of a foundation to build other work on that will let one rigorously verify the correct use, optimization, or compilation of LLVM programs. The link below has lots of examples of awesome work in security or verification that builds on LLVM. Some use Vellvm already with most able to benefit somehow if work is put in.
IR ("intermediate representation") is the form of code inside the compiler, used after a frontend (like clang or rustc) has parsed and made sense of the particular language you're using, and before the compiler is ready to output native code. All optimizations happen on IR.
So, this is a toolkit for letting you prove the correctness of LLVM optimizations (i.e., that some IR has the same behavior before and after the optimization pass), so that an optimization pass isn't accidentally introducing exploitable bugs into your software.