[clang] [llvm] [SPIR-V] Add SPIR-V structurizer (PR #107408)

[Nathan Gauër via cfe-commits]

Keenuts wrote:

Thanks! No worries!
Applied the few changes. And yes, there will be iterations on this. For example, merge-exit pass is not completely finished, and other specific scenarios. But this will be done through smaller PRs.
I also need to look into how to test this since people want me to revert the spirv-sim utility.

https://github.com/llvm/llvm-project/pull/107408