[clang] [llvm] [HLSL] Adding Flatten and Branch if attributes (PR #116331)

[Nathan Gauër via cfe-commits]

https://github.com/Keenuts commented:

Thanks, answered the question for the intrinsic.
As Chris said, this would definitely needs to have some tests in the spir-v backend.
Especially given that the structurizer assumes the selection control mask was always `None`.
When splitting edges, or moving condition, we should make sure the mask is correctly moved/copied, etc

https://github.com/llvm/llvm-project/pull/116331