[llvm] [NFC][X86] Add getMemoryOperandIdx helper function (PR #211685)

[Zachary Yedidia via llvm-commits]

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zyedidia wrote:

Yes it looks like the previous code relies on the caller passing the result of `getOperandBias` to this function as `CurOp`. I think this function could be refactored to not take a `CurOp` at all, and do all the `CurOp` movement in the caller (`encodeInstruction`) instead. I could do that as a separate PR after this?

https://github.com/llvm/llvm-project/pull/211685