[llvm] [RISCV] Fix the disassembler's handling of C.LUI when imm=0 (PR #133450)

[Paul Bowen-Huggett via llvm-commits]

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paulhuggett wrote:

Yes, makes sense.

I don't know how I managed to miss that as a home for disassembler tests. I've also changed the extension to .txt because it seems like a better choice and matches the other tests there.

Thank you.

https://github.com/llvm/llvm-project/pull/133450