[llvm] [DirectX] Add `WaveActiveOp` builtin (PR #112058)

[Chris B via llvm-commits]

llvm-beanz wrote:

+1 on @bogner's comment. DXC's use of "opcodes" in initial IR generation made it extremely difficult to read the intermediate (and final) IR, which also made it difficult to write and maintain tests. In our case this will also make it difficult for other frontends to target DirectX because they'll also need our magic decoder rings.

https://github.com/llvm/llvm-project/pull/112058