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

[Finn Plummer via llvm-commits]

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inbelic wrote:

Does this test also check that `removeDeadConstantUsers` is invoked as we expect? If so how?

If not, can we add a test case that demonstrates it works as expected. Also for the other passes where it was introduced

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