[llvm] [llvm-mca] Add optional identifier field to mca::Instruction (PR #97867)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> As you can see, the same instruction might have different metadata at different points in the program. For recycled instructions, we ensure to tag them appropriately when they are [instantiated](https://github.com/securesystemslab/LLVM-MCA-Daemon/blob/llvm-main/MCAWorker.cpp#L412). Having a unique identifier to refer to instructions in the pipeline seems useful to me.

Could you please help me understand what new functionality this would allow in MCAD? It looks like we have a way to [identify](https://github.com/securesystemslab/LLVM-MCA-Daemon/blob/llvm-main/MCAWorker.cpp#L412) them.

https://github.com/llvm/llvm-project/pull/97867