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

[Justin Bogner via llvm-commits]

bogner wrote:

> @bogner should we hold off on this pr?

Let's go ahead and get this change in. The `Load/Store operand is not a pointer type` are masking any cases where this comes up in DML, but that doesn't change the fact that we do need to match our linkage rules for functions and clean up arbitrary globals here.

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