[llvm] MachineCSE: Remove check for subreg on a def operand (PR #124095)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jan 23, 9:33 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/124095).


https://github.com/llvm/llvm-project/pull/124095