[llvm] [GlobalISel] Add `combine` action for C++ combine rules (PR #135941)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

### Merge activity

* **Apr 25, 6:08 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/135941).


https://github.com/llvm/llvm-project/pull/135941