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

[Rahul Joshi via llvm-commits]

jurahul wrote:

I did not do any measurements as it seemed simple to implement. The only benefit will be a minor reduction in code size as this eliminates dead code from each target's disassembler code. I suppose `OPC_SoftFail` could also use such a treatment. I can measure the reduction in size of llvm-mc with/without but I suspect it will be small.

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