[Coverage] Introduce getBranchCounterPair(). NFC. (#112702)

This aggregates the generation of branch counter pair as `ExecCnt` and
`SkipCnt`, to aggregate `CounterExpr::subtract`. At the moment:

- This change preserves the behavior of
`llvm::EnableSingleByteCoverage`. Almost of SingleByteCoverage will be
cleaned up by coming commits.

- `IsCounterEqual(Out, Par)` is introduced instead of
`Counter::operator==`. Tweaks would be required for the comparison for
additional counters.


https://discourse.llvm.org/t/rfc-integrating-singlebytecoverage-with-branch-coverage/82492

Author: chapuni