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

[Finn Plummer via llvm-commits]

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inbelic wrote:

Also, if we have two successive extract (or insert) elements, do we want to ensure that they will both index from the same newly allocated array? Or is it safe to assume that the un-required extra allocations of the array will get cleaned up?

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