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

[Farzon Lotfi 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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farzonl wrote:

so  if you don't add a vector test then you can remove the `-scalarizer` flag.  

While a vector test would overlap with  the `atan` vector tests. Its benefit is that this unit tests could also be considered an integration test.

Its a small benefit so I don't have a strong opinion here since we have been using `opt` to do our tests. That said I'll signal here that I think long term it would better for us to use `llc` for tests cases like this one where we want to see the final transformation and not just the per pass transformations.

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