[llvm] RegAllocFast: Avoid using temporary DiagnosticInfo (PR #120184)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Dec 17, 4:16 AM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/120184).


https://github.com/llvm/llvm-project/pull/120184