[llvm] [SPIR-V] add convergence region analysis (PR #78456)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

@michalpaszkowski I was able to build the next steps of the structurizer. So if you are OK, I'm fine to merge this analysis.


The issue I had was:
 - I needed to generate `OpLoopMerge` instruction in the backend, hence working on MIR.
 - But convergence intrinsics were only available at the IR level.
 - Convergence intrinsics could not be lowered down to MIR, meaning I needed to strip them.
 - This made this analysis useless.
 
 But I found this PR: https://github.com/llvm/llvm-project/pull/67006
 Rebasing my structurizer on it solved all my issues: since I can now keep the intrinsics in the IR, I can do the proper work in the backend, as expected.

https://github.com/llvm/llvm-project/pull/78456