a 6502 backend for LLVM has been attempted a couple times[1][2], but the fact that the 6502 only has three registers imposes severe limitations w.r.t. LLVM's calling conventions.
You would probably want to add some tricks directly there, maybe register renaming (I don't know if LLVM does "variable renaming", let's put it this way)
[1] https://github.com/c64scene-ar/llvm-6502
[2] https://github.com/beholdnec/llvm-m6502