[llvm] [nfc] Improve testability of PGOInstrumentationGen (PR #104490)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

### Merge activity

* **Aug 16, 12:44 PM EDT**: @mtrofin started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/104490).


https://github.com/llvm/llvm-project/pull/104490