[llvm] RegAllocGreedy: Fix use after free during last chance recoloring (PR #120697)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jan 6, 11:12 AM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/120697).


https://github.com/llvm/llvm-project/pull/120697