[llvm] [DirectX] Add atan2 intrinsic and expand for DXIL backend (p1) (PR #108865)

[Tex Riddell via llvm-commits]

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+; RUN: opt -S -dxil-op-lower -mtriple=dxil-pc-shadermodel6.3-library %s | FileCheck %s
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+; Make sure correct dxil expansions for atan2 are generated for float and half.
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tex3d wrote:

I added a vector test.  Let me know if this looks good now.

https://github.com/llvm/llvm-project/pull/108865