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

[James Y Knight via llvm-commits]

https://github.com/jyknight commented:

I think this is a good cleanup, just some minor comments.

One thing I've wanted to do for a long time, but never got around to:
I think we should move the long blobs of helper functions out of this file, and into a separate header which gets included from the output.

I don't think there's any real value in having them be emitted textually into the output code, vs included, and it'll be a lot easier to read/edit that way.

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