[llvm] [BOLT] Introduce helpers to match `MCInst`s one at a time (NFC) (PR #138883)

[Anatoly Trosinenko via llvm-commits]

https://github.com/atrosinenko commented:

I'm not sure it is feasible to me to unify the two approaches right now, thus  I tried to summarize the reasons not to do so in the description of the namespace :)

I renamed my namespace to `LowLevelInstMatcherDSL`, but kept it in MCInstUtils.cpp instead of MCPlusBuilder.cpp so far. My idea was that it is probably more expected by the user to find such utilities in a dedicated source file instead of the file defining target-specific callbacks. Honestly, I would rather move the existing MCInstMatcher to MCInstUtils.cpp, but even if it would be an acceptable solution, it should probably be done in a separate PR.

https://github.com/llvm/llvm-project/pull/138883