[llvm] [SPIR-V] Add SPIR-V structurizer (PR #97606)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

Closing this PR for now.
Rewrite the structurizer to work from the merge nodes, and also handle some cases with switches and fallthrough. Some issues I encountered outweighed the small benefit I had to make this as the MIR level instead of the IR level. The next PR will be a complete rewrite at the IR level.
Closing this one as a result. 

https://github.com/llvm/llvm-project/pull/97606