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

[Zachary Yedidia via llvm-commits]

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zyedidia wrote:

If the performance of the check is a concern, perhaps it is simpler to leave cases that don't do the check as they were and just not touch them, rather than introducing two helpers? Let me know what you prefer.

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