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

[Jason Eckhardt via llvm-commits]

nvjle wrote:

> > 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.
> 
> Sorry, I meant to add: I didn't mean that as a requirement for this PR, just a suggestion for potential future work.

Thank you for the review, I've attended to the items mentioned.
Also, I do like your idea for future work and may take it up myself at some point.

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