[llvm] [ConstantFolding] Add support for llvm.atan in constant folding. (PR #143416)

[Joshua Cranmer via llvm-commits]

jcranmer-intel wrote:

`atan(-0.0)` is `-0.0` per IEEE 754, so it's a problem if it's being folded to `+0.0`.

https://github.com/llvm/llvm-project/pull/143416