[llvm] [DirectX] Add GlobalDCE pass after finalize linkage pass in DirectX backend (PR #151071)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

I did a run of the DML Shaders today. There are currently only two validator errors left and none of them are `Internal declaration is unused` . I'm conflicted now on moving forward with this PR.  

The remaining two validation issues:
```
40x error: Bitcast on struct types is not allowed.
32x error: Pointer type bitcast must be have same size.
```

@Icohedron are you aware of any changes that went in that could have impacted these validation issues? Is it possible the bitcast errors are masking the unused errors?

@bogner should we hold off on this pr?


https://github.com/llvm/llvm-project/pull/151071