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

[Nathan Gauër via llvm-commits]

Keenuts wrote:

Had to rename the pass and move it outside of the SPIRV namespace.
This was because to use the pass with the old pass manager (hence this backend), I need to use the INITIALIZE_PASS defines.
Those defines require the namespace around the pass to be `llvm::` AFAIK.
moved the passes to the global llvm namespace, while keeping 'internal' stuff and structures in the SPIRV namespace.
Because I moved it to the shared namespace, I prefixed the passes with the `SPIRV` prefix.

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