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

[Krzysztof Parzyszek via llvm-commits]

https://github.com/kparzysz commented:

This makes sense to me, but I'd wait for someone else to review this to see if it meets their needs.

A couple of suggestions regarding style:
- use structured bindings in place of "auto &KV : some-map",
- modify "append" to take "const std::string &", it would get rid of the "c_str" and make the code easier to read.

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