[llvm] [X86] Add MI-layer routine for getting the index of the first address operand. NFC (PR #78019)

[Shengchen Kan via llvm-commits]

KanRobert wrote:

> it does not handle pseudo-instructions like TCRETURNmi, resulting in missed optimizations (e.g., x86-optimize-LEAs)

Why there is no test update for this?

https://github.com/llvm/llvm-project/pull/78019