[llvm] [IR] Add llvm.sincospi intrinsic (PR #125873)

[Benjamin Maxwell via llvm-commits]

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MacDue wrote:

This PR depends on https://github.com/llvm/llvm-project/pull/121948 which adds `llvm.modf`, so only the top commit is releated to `sincospi`.

https://github.com/llvm/llvm-project/pull/125873