[llvm] [MCA] Enable customization of individual instructions (PR #155420)

[Roman Belenov via llvm-commits]

r-belenov wrote:

> That being said, I still don't think we should have the Instrument::customize and Instrument::canCustomize -- I believe we can conditionally static_cast to this new LatencyInstrument by the getDesc it returns.
We still need callbacks in InstrBuilder to check whether some instrument can customize InstrDesc (so we cannot reuse the description) and to do the customization, and latency change is not the only possible customization (though it is the only one in the current patch), so I don't like having the code in InstrumentManager that checks for specific instruments.

https://github.com/llvm/llvm-project/pull/155420