[llvm] PeepholeOpt: Handle subregister compose when looking through reg_sequence (PR #127051)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Feb 17, 8:05 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/127051).


https://github.com/llvm/llvm-project/pull/127051