[llvm] [TableGen][DecoderEmitter] Remove dead OPC_Fail (PR #155229)

[Rahul Joshi via llvm-commits]

https://github.com/jurahul approved this pull request.

LGTM. I left the OPC_Fail when I added the OrFail variants in case there was a path in the code that escaped this check (to generate OrFail variant), but yeah, looks like there is none. Thanks for cleaning this up.

https://github.com/llvm/llvm-project/pull/155229