[llvm] [TableGen][DecoderEmitter] Resolve a FIXME in emitDecoder (PR #154649)

[Min-Yih Hsu via llvm-commits]

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

LGTM, thanks. I can add a disassembler test in another commit later

https://github.com/llvm/llvm-project/pull/154649