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

[Farzon Lotfi via llvm-commits]

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farzonl wrote:

Moved to new file. added `REQUIRES: asserts`.  Assuming i didn't notice because i'm on a debug build.

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