LLVM intermediate representation and Mill code are going to be pretty different. The LLVM machine model is a register based machine (with an arbitrary number of registers--the backends do the work of register allocation). Basically, an easier RISC-ish assembly.

So, while LLVM would be helpful for porting things to the Mill, as it's largely a "solve once use everywhere" problem, it's still not trivial. It could take a lot of effort to make it competitive.