[llvm] [GlobalISel] Fix silently dropped MIFlags on selected instructions (PR #138851)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

### Merge activity

* **May 7, 8:06 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/138851).


https://github.com/llvm/llvm-project/pull/138851