[llvm] [RISCV] Remove unnecessary if guard before calling SignExtend64<6> in decodeCLUIImmOperand. NFC (PR #133514)

[Paul Bowen-Huggett via llvm-commits]

paulhuggett wrote:

I tried adding an assertion to verify that this test has no effect. It did not trip despite the c_lui_disasm.txt exercising all possible inputs. LGTM.

https://github.com/llvm/llvm-project/pull/133514