[flang] [llvm] [flang] Optimize `acospi` precision (PR #152869)

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c8ef wrote:

### Merge activity

* **Aug 11, 4:15 PM UTC**: [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/152869) rebased this pull request as part of a merge.


https://github.com/llvm/llvm-project/pull/152869