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

[Lei Huang via llvm-commits]

lei137 wrote:

I would like to xfail this tc for now so we can bring the aix bot back to green.
https://github.com/llvm/llvm-project/pull/143723

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