[llvm] PeepholeOpt: Allow introducing subregister uses on reg_sequence (PR #127052)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Feb 21, 9:15 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/127052).


https://github.com/llvm/llvm-project/pull/127052