[llvm] RegisterCoalescer: Set undef on full register uses when coalescing implicit_def (PR #118321)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Dec 2, 2:42 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/118321).


https://github.com/llvm/llvm-project/pull/118321