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

[Nathan Gauër via llvm-commits]

https://github.com/Keenuts approved this pull request.

LGTM on the SPIR-V side. Just a superfluous loop thing.
For the structurizer, seems like this is OK, worst case we might end-up with a new condition which has no Flatten/Dontflatten attachment but should be fine.

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