[llvm] [TableGen] fix tlbgen for EncodingByHwMode (PR #84906)

[via llvm-commits]

superZWT123 wrote:

Hi [nvjle](https://github.com/nvjle), [kparzysz](https://github.com/kparzysz). I might take a few days to organize my patch and perform some more in-depth checks (such as enabling the EncodingByHwMode functionality on other backends like RISCV, and build tablegen in debug mode to dump some records) to ensure the functionality is correct, although we have already conducted numerous CI tests on this patch in our local project. I will try to decouple the code of 'DecoderEmitter' part within one or two days to submit a pull request first. Thank you very much for your patient review and valuable comments.

https://github.com/llvm/llvm-project/pull/84906