[llvm] [DirectX] Scalarize `extractelement` and `insertelement` with dynamic indices (PR #141676)

[Finn Plummer via llvm-commits]

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inbelic wrote:

Does it make sense to have a multi-dimension testscase, `<2 x <3 x float>>`?

https://github.com/llvm/llvm-project/pull/141676