[llvm] [TableGen] DecoderEmitter clean-ups and modernization. (PR #84832)

[Wang Pengcheng via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

I think it LGTM, but I don't know if this kind of rewrites is encouraged.
cc @topperc 

https://github.com/llvm/llvm-project/pull/84832