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

[Chinmay Deshpande via llvm-commits]

chinmaydd wrote:

Hi @boomanaiden154, thanks for your comment. We are using llvm-mca as a library in [MCAD](https://github.com/securesystemslab/LLVM-MCA-Daemon). MCAD tries to tweak the timings of individual instructions based on dynamic information from an emulator-like environment (if available). For instance, we have basic support for identifying aliasing loads and stores based on information retrieved from QEMU.

As you can see, the same instruction might have different metadata at different points in the program. Even 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 more.

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