[llvm] [LegacyPM][DirectX] Add legacy scalarizer back for use in the DirectX backend (PR #107427)

[Justin Bogner via llvm-commits]

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bogner wrote:

I think it'd be better to just add a `llc-pipeline.ll` test like in AMDGPU to check the pass structure (and that it includes the scalarizer) and not include the checks for scalarizing running. The test is generally useful for our other passes as well.

Also, does this pass need a `REQUIRES: asserts`? The similar tests in other backends have it.

https://github.com/llvm/llvm-project/pull/107427