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

[Deric C. via llvm-commits]

Icohedron wrote:

> Rest of the code looks good. I'm only concered about how we are handling ConstExpr. I feel like the way we are doing it isn't future proofing to the next pass that might operate on them and not know to delete them after instruction erasure.

I changed the handling of ConstantExpr. Each pass no longer has the responsibility of cleaning up dead ConstantExprs.
Now the PointerTypeAnalysis pass will call `removeDeadConstantUsers` on globals before checking its users.

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