[llvm] [Exegesis][RISCV] Add RISCV support for llvm-exegesis (PR #89047)

[Clement Courbet via llvm-commits]

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@@ -35,6 +35,8 @@ struct InstructionTemplate {
   const MCOperand &getValueFor(const Variable &Var) const;
   MCOperand &getValueFor(const Operand &Op);
   const MCOperand &getValueFor(const Operand &Op) const;
+  MCOperand &getValueFor(unsigned OpIdx);
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legrosbuffle wrote:

Having the same name for this overload is confusing: the two overloads for `Variable` and `Operand` are unambiguous thanks to the types, but this one is unclear as to whether we're talking about an operand or variable.  `getValueForOperandIdx` ?

https://github.com/llvm/llvm-project/pull/89047