[llvm] SystemZ: Handle copies between gr64 and fp64 (PR #124890)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jan 29, 11:04 PM EST**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/124890).


https://github.com/llvm/llvm-project/pull/124890