[clang] [Clang] Change ballot mask handling for GPU intrinsics (PR #176202)

[Johannes Doerfert via cfe-commits]

jdoerfert wrote:

I don't get this change. If the user wants the behaviour you have after the PR, they can simply pass in ~0 as lane mask. If they want to restrict the ballot result to a subset of lanes, they can pass in something else. What is the problem here?

https://github.com/llvm/llvm-project/pull/176202