[llvm] [DirectX] Remove intrinsic definitions with no use (PR #133459)

[Justin Bogner via llvm-commits]

bogner wrote:

A couple of concerns here:
1. Why are we using the DXILLegalizer for this? It's fairly unrelated to the other work it's doing and changing this to a module pass isn't really saving us anything. This could be a separate pass for absolutely no cost.
2. This is a pretty big hammer. Why are these intrinsic definitions floating around? I feel like this is likely to just hide bugs in other passes and should probably be targeted to only intrinsics that have a good reason to have a dead definition sticking around, if there are any.
3. It's surprising that this change didn't need an update to `test/CodeGen/DirectX/llc-pipeline.ll

https://github.com/llvm/llvm-project/pull/133459