[llvm] [RISCV] Rename some DecoderNamespaces and cleanup debug messages. NFC (PR #131409)

[Alex Bradbury via llvm-commits]

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

I've left some nitpitcks on the debug message cleanups. But otherwise this LGTM.

https://github.com/llvm/llvm-project/pull/131409