How hard would it be to create a new backend in D?

[rempas]

I was wondering how easy it would be to create a new backend in 
D? From what I know all of the tree DMD, LDC and GDC are using 
the same frontend which is the one from DMD. So I would suppose 
that there is a way to do that and that probably DMD has evolved 
over the years to make its frontend more portable. However, how 
strain-forward is it? Is it legit to do it or will it require 
"hacking" (if you understand what I mean). So it's not about 
working hard but if the work makes sense or if I'll constantly 
find obstacles that no one would be able to help me with.

I know some of you will ask so, I want to use 
[mir](https://github.com/vnmakarov/mir) as a backend because of 
it's fast compile times and it's great runtime performance! So 
yeah, not a question purely out of curiosity!