[llvm] [NFC][PGO] Factor downscaling of branch weights out of `Instrumentation` into `ProfileData` (PR #153735)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

### Merge activity

* **Aug 15, 5:17 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/153735).


https://github.com/llvm/llvm-project/pull/153735