[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
+
+; Make sure correct dxil expansions for atan2 are generated for float and half.
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tex3d wrote:

I added the expansion-only test.  I experimented with -scalarizer for vector tests, but since this just expands to atan and other instructions, it's just testing scalarization of the atan intrinsic, which is already tested elsewhere.  I can add a vector expansion-only test, if you think that's useful (I have one, but didn't add it here yet).

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