[llvm] [NFCI][TableGen][DecoderEmitter] Cull `TryDecode` handling when possible (PR #142974)

[Rahul Joshi via llvm-commits]

jurahul wrote:

Thanks. I have a new version that culls more with which I got the above results. Will ask for another review once that passes checks.

https://github.com/llvm/llvm-project/pull/142974