[llvm] [BOLT] Modify MCPlus annotation internals. NFCI. (PR #70412)

[Job Noorman via llvm-commits]

https://github.com/mtvec approved this pull request.

LGTM in general, just two remarks:
- There is some duplication of logic between `getNumPrimeOperands` and `getAnnotationInstOp` (searching for `kInst`). Maybe one could be written in function of the other?
- Since we don't need any `MCPlusBuilder` state anymore for annotations, should we move all the annotation-related functions to `MCPlus`? Not saying that should happen in this PR (the diff is probably fairly large) but it seems to be the more logical place for this functionality.

https://github.com/llvm/llvm-project/pull/70412