[llvm] [SPIR-V] Add pass to merge convergence region exit targets (PR #92531)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

> `auto *RI = dyn_cast` when `RI` is unused causes -Wunused-but-set-variable warnings with a relative new Clang (>= 2021-04). I fixed them in [a088c61](https://github.com/llvm/llvm-project/commit/a088c61d5c409ec0b8994340866d5864ba913516).
> 
> You probably want to compile and test a modern Clang to catch such issues.

Oh, thanks for fixing!
Weirdly my system's clang is 16.0.6, so seems like it's from 2023.
Added '-DCMAKE_CXX_FLAGS=-Wunused-but-set-variable" but this warning is still not showing on main (before your PR of course). With what options do you compile?

Mine are:
```
cmake -Hllvm -GNinja -Bbuild -DCMAKE_BUILD_TYPE=Debug -DLLVM_ENABLE_PROJECTS='clang' -DLLVM_TARGETS_TO_BUILD='X86' -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD="DirectX;SPIRV" -DLLVM_OPTIMIZED_TABLEGEN=1 -DLLVM_ENABLE_LLD=1 -DLLVM_USE_SPLIT_DWARF=1 -DCMAKE_CXX_COMPILER=clang++-16 -DCMAKE_C_COMPILER=clang-16 -DCMAKE_EXPORT_COMPILE_COMMANDS=1 -DCMAKE_INSTALL_PREFIX=build/install -DLLVM_INCLUDE_SPIRV_TOOLS_TESTS=1
```

https://github.com/llvm/llvm-project/pull/92531