[llvm] X86: Remove hack in shouldRewriteCopySrc for subregister handling (PR #125224)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Feb 5, 11:06 AM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/125224).


https://github.com/llvm/llvm-project/pull/125224