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

[Chris B via llvm-commits]

https://github.com/llvm-beanz requested changes to this pull request.

Mostly minor nits, but the stuff in `SPIRVStructurizerWrapper::run` has to change before we land this.

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