[llvm] [DirectX] Fix the writing of ConstantExpr GEPs to DXIL bitcode (PR #154446)

[Deric C. via llvm-commits]

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Icohedron wrote:

I just added another function to this dxil-dis test to exercise the `removeDeadConstantUsers`. The test now fails if `removeDeadConstantUsers` is not called.

https://github.com/llvm/llvm-project/pull/154446