[llvm] DAG: Use fast variants of fast math libcalls (PR #147481)

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **Jul 13, 1:37 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/147481).


https://github.com/llvm/llvm-project/pull/147481