[llvm] [ctx_prof] Handle `select` and its `step` instrumentation (PR #109185)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

### Merge activity

* **Sep 23, 6:20 PM EDT**: @mtrofin started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/109185).


https://github.com/llvm/llvm-project/pull/109185