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

[Deric C. via llvm-commits]

Icohedron wrote:

It just came to me that the implementation still needs more work. Two issues:
1. Two `insertelement` instructions on the same vector currently is incorrect because they affect each other's results, which shouldn't be the case.
2. If `extractelement` or `insertelement` is used on a vector that is not a function arg, then the stores to the alloca are invalid because they reference a vector that is not yet defined.

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