[llvm] [AVR] Remove some custom decoder functions (NFCI) (PR #156852)

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

I'll do more in subseqent patches to keep this one smaller.
I've spotted at least one bug and a few instruction encodings need to be refactored.


https://github.com/llvm/llvm-project/pull/156852