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

[Deric C. via llvm-commits]

Icohedron wrote:

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

Also there is a third remaining validation issue: 120 occurrences of `Load/Store operand is not a pointer type`

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