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

[Justin Bogner via llvm-commits]

https://github.com/bogner approved this pull request.

Looks good pending agreement that we're good to bring this back and my comment about the pipeline test.

Arguably the various bits of test cleanup could be committed separately, but it doesn't matter too much.

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