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

[Jason Eckhardt via llvm-commits]

nvjle wrote:

If one of the selected reviewers would kindly review this patch, I'd appreciate it.

While this is technically NFC, I'd just like to be sure others agree with my use of more modern raw strings for the large emitted code fragments. This makes them far more readable (and modifiable) than the old-school strings-- since they look just like ordinary code (no quoting needed, no per-line stream operators, etc).

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