[llvm] [RISCV][NFCI] Rationalize Immediate Definitions (PR #120718)

[Yingwei Zheng via llvm-commits]

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@@ -277,7 +283,7 @@ class UImm20Operand : RISCVOp {
   let OperandType = "OPERAND_UIMM20";
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dtcxzyw wrote:

`UImm20Operand` is unused now.

https://github.com/llvm/llvm-project/pull/120718